Ageing (British and Australian English) or aging (American and Canadian English) is the accumulation of changes in an organism or object over time.^[1] Ageing in humans refers to a multidimensional process of physical, psychological, and social change. Some dimensions of ageing grow and expand over time, while others decline. Reaction time, for example, may slow with age, while knowledge of world events and wisdom may expand. Research shows that even late in life potential exists for physical, mental, and social growth and development. Ageing is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions. Age is usually measured in full years — and months for young children. A person's birthday is often an important event. Roughly 100,000 people worldwide die each day of age-related causes.^[2]

The term "ageing" is somewhat ambiguous. Distinctions may be made between "universal ageing" (age changes that all people share) and "probabilistic ageing" (age changes that may happen to some, but not all people as they grow older, such as the onset of type two diabetes Diabetes mellitus type 2 or type 2 diabetes (formerly called non -insulin-dependent diabetes mellitus , or adult-onset diabetes) is a disorder that is characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency. Diabetes is often initially managed by increasing exercise and dietary modification. As the). Chronological ageing, referring to how old a person is, is arguably the most straightforward definition of ageing and may be distinguished from "social ageing" (society's expectations of how people should act as they grow older) and "biological ageing" (an organism's physical state as it ages). There is also a distinction between "proximal ageing" (age-based effects that come about because of factors in the recent past) and "distal ageing" (age-based differences that can be traced back to a cause early in person's life, such as childhood poliomyelitis Poliomyelitis, often called polio or infantile paralysis, is an acute viral infectious disease spread from person to person, primarily via the fecal-oral route. The term derives from the Greek poliós , meaning "grey", myelós (µυελός), referring to the "spinal cord", and the suffix -itis, which denotes inflammation).^[3]

Differences are sometimes made between populations of elderly people. Divisions are sometimes made between the young old (65–74), the middle old (75–84) and the oldest old (85+). However, problematic in this is that chronological age does not correlate perfectly with functional age, i.e. two people may be of the same age, but differ in their mental and physical capacities. Each nation, government and non-government organization has different ways of classifying age.

Population ageing Population ageing or population aging occurs when the median age of a country or region rises. With the exception of 18 countries termed by the United Nations 'demographic outliers' (see the Ud 2005 Human Development Report) this process is taking place in every country and region across the globe is the increase in the number and proportion of older people in society. Population ageing has three possible causes: migration, longer life expectancy Life expectancy is the expected number of years of life remaining at a given age. It is denoted by ex, which means the average number of subsequent years of life for someone now aged x, according to a particular mortality experience. (In technical literature, this symbol means the average number of complete years of life remaining, ie excluding (decreased death rate), and decreased birth rate. Ageing has a significant impact on society. Young people tend to commit most crimes, they are more likely to push for political and social change, to develop and adopt new technologies, and to need education. Older people have different requirements from society and government as opposed to young people, and frequently differing values as well. Older people are also far more likely to vote, and in many countries the young are forbidden from voting. Thus, the aged have comparatively more political influence.

Senescence

In biology, senescence Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to that of the whole organism. There are a number of theories as to why senescence occurs, including ones that claim it is programmed by gene expression changes and that it is the state or process of ageing. Cellular senescence is a phenomenon where isolated cells demonstrate a limited ability to divide in culture (the Hayflick Limit The Hayflick limit is the number of times a normal cell population will divide before it stops, presumably because the telomeres reach a critical length, discovered by Leonard Hayflick in 1961), while organismal senescence is the ageing of organisms. After a period of near perfect renewal (in humans, between 20 and 35 years of age), organismal senescence is characterized by the declining ability to respond to stress Stress is a term in psychology and biology, first coined in the 1930s, which has in more recent decades become a commonplace of popular parlance. It refers to the consequence of the failure of an organism – human or animal – to respond appropriately to emotional or physical threats, whether actual or imagined, increasing homeostatic Homeostasis is the property of a system, either open or closed, that regulates its internal environment and tends to maintain a stable, constant condition. Typically used to refer to a living organism, the concept came from that of milieu interieur that was created by Claude Bernard and published in 1865. Multiple dynamic equilibrium adjustment imbalance and increased risk of disease. This currently irreversible series of changes inevitably ends in death Death is the termination of the biological functions that define a living organism. The word refers both to a particular process and to the condition that results thereby. The nature of the latter has been for millennia a central concern of the world's religious traditions and of philosophical enquiry. Belief in some kind of afterlife or rebirth. Some researchers (specifically biogerontologists Gerontology is the study of the social, psychological and biological aspects of aging. It is distinguished from geriatrics, which is the branch of medicine that studies the disease of the elderly) are treating ageing as a disease. As genes that have an effect on ageing are discovered, ageing is increasingly being regarded in a similar fashion to other geneticly influenced "conditions", potentially "treatable."

Indeed, ageing is not an unavoidable property of life. Instead, it is the result of a genetic program. Numerous species show very low signs of ageing ("negligible senescence"), the best known being trees like the bristlecone pine The bristlecone pines are a small group of pine trees that are thought to reach an age far greater than that of any other single living organism known, up to nearly 5,000 years (however Dr. Hayflick states that the bristlecone pine has no cells older than 30 years), fish like the sturgeon Sturgeon is the common name used for some 26 species of fish in the family Acipenseridae, including the genera Acipenser, Huso, Scaphirhynchus and Pseudoscaphirhynchus. The term includes over 20 species commonly referred to as sturgeon and several closely related species that have distinct common names, notably sterlet, kaluga and beluga and the rockfish Sebastes is a genus of fish in the family Sebastidae , most of which have the common name of rockfish. Most of the world's almost 110 rockfish species live in the north Pacific, although two species (S. capensis and S. oculatus) live in the South Pacific/Atlantic and four (S. fasciatus, S. marinus, S. mentella and S. viviparus) live in the north, invertebrates like the quahog The hard clam , also known as a quahog (or quahaug), round clam, or hard-shell (or hard-shelled) clam, is an edible marine bivalve mollusk which is native to the eastern shores of North America, from Prince Edward Island to the Yucatán Peninsula. It is one of many unrelated edible bivalves which in the USA are frequently referred to simply as and sea anemone Sea anemones are a group of water dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. As cnidarians, sea anemones are closely related to corals, jellyfish, tube-dwelling anemones, and Hydra^[4] and lobster Clawed lobsters comprise a family of large marine crustaceans. Lobsters are economically important as seafood, forming the basis of a global industry that nets more than US$1 billion annually.^[5][6]

In humans and other animals, cellular senescence Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to that of the whole organism. There are a number of theories as to why senescence occurs, including ones that claim it is programmed by gene expression changes and that it has been attributed to the shortening of telomeres A telomere is a region of repetitive DNA at the end of a chromosome, which protects the end of the chromosome from deterioration. Its name is derived from the Greek nouns telos "end" and merοs (μέρος, root: μερ-) "part" with each cell cycle The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus (prokaryotic), the cell cycle occurs via a process termed binary fission. In cells with a nucleus (eukaryotes), the cell cycle can be divided in two brief periods: interphase—during which; when telomeres become too short, the cells die. The length of telomeres is therefore the "molecular clock," predicted by Hayflick Leonard Hayflick , Ph.D., is Professor of Anatomy at the University of California, San Francisco, School of Medicine, and was Professor of Medical Microbiology at Stanford University School of Medicine. He is a past president of the Gerontological Society of America and was a founding member of the council of the National Institute on Aging (NIA).

Telomere length is maintained in immortal cells (e.g. germ cells In biology, germ cells are the cells that give rise to the gametes of organisms that reproduce sexually. In many animals, the germ cells originate near the gut and migrate to the developing gonads. There, they undergo cell division of two types, mitosis and meiosis, followed by cellular differentiation into mature gametes, either eggs or sperm and keratinocyte Keratinocytes are the predominant cell type in the epidermis, the outermost layer of the human skin, constituting 95% of the cells found there. Those keratinocytes found in the basal layer of the skin are sometimes referred to as "basal cells" or "basal keratinocytes". The primary function of keratinocytes is the formation of stem cells, but not other skin cell types) by the telomerase enzyme. In the laboratory, mortal cell lines can be immortalized by the activation of their telomerase gene, present in all cells but active in few cell types. Cancerous Cancer /ˈkænsər/ (medical term: malignant neoplasm) is a class of diseases in which a group of cells display uncontrolled growth (division beyond the normal limits), invasion (intrusion on and destruction of adjacent tissues), and sometimes metastasis (spread to other locations in the body via lymph or blood). These three malignant properties cells must become immortal to multiply without limit. This important step towards carcinogenesis implies, in 85% of cancers, the reactivation of their telomerase gene by mutation. Since this mutation is rare, the telomere "clock" can be seen as a protective mechanism against cancer.^[7] Research has shown that the clock must be located in the nucleus of each cell and there have been reports that the longevity clock might be located in genes on either the first or fourth chromosome of the twenty-three pairs of human chromosomes.

Other genes are known to affect the ageing process, the sirtuin Silent Information Regulator Two proteins, or sirtuins, are a class of proteins that possess either histone deacetylase or mono-ribosyltransferase activity and are found in organisms ranging from bacteria to humans. Named after the yeast silent mating-type information regulation two, the gene responsible for cellular regulation in yeast, sirtuins family of genes have been shown to have a significant effect on the lifespan of yeast Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with the 1,500 species currently described estimated to be only 1% of all yeast species. Most reproduce asexually by budding, although a few do so by binary fission. Yeasts are unicellular, although some species with yeast forms may become multicellular through the formation of and nematodes The nematodes or roundworms (phylum Nematoda) are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated[weasel words] that the total number of described and undescribed. Over-expression of the RAS2 gene increases lifespan in yeast substantially.

In addition to genetic ties to lifespan, diet has been shown to substantially affect lifespan in many animals. Specifically, caloric restriction Calorie restriction, or caloric restriction , is a dietary regimen that restricts calorie intake, where the baseline for the restriction varies, usually being the previous, unrestricted, intake of the subjects. CR when not associated with malnutrition, is thought to improve age-related health and to slow the aging process in some animals and fungi (that is, restricting calories to 30-50% less than an ad libitum Ad libitum is Latin for "at [one's] pleasure"; it is often shortened to "ad lib" or "ad-lib" (as a verb or noun). The roughly synonymous phrase a bene placito ("at [one's] good pleasure") is less common but, in its Italian form a piacere, entered the musical lingua franca (see below) animal would consume, while still maintaining proper nutrient intake), has been shown to increase lifespan in mice up to 50%. Caloric restriction works on many other species beyond mice (including species as diverse as yeast and Drosophila), and appears (though the data is not conclusive) to increase lifespan in primates according to a study done on Rhesus monkeys at the National Institute of Health (US), although the increase in lifespan is only notable if the caloric restriction is started early in life. Since, at the molecular level, age is counted not as time but as the number of cell doublings, this effect of calorie reduction could be mediated by the slowing of cellular growth and, therefore, the lengthening of the time between cell divisions.

Drug companies are currently searching for ways to mimic the lifespan-extending effects of caloric restriction without having to severely reduce food consumption.

In his book, 'How and Why We Age', Dr. Hayflick notes a contradiction to the caloric restriction longevity increase theory for humans, noting that data from the Baltimore Longitudinal Study of Ageing show that being thin does not favour longevity.

Dividing the lifespan

Both a human's and an animal's life is often divided into various ages. However, because biological changes are slow moving and vary from person to person, arbitrary dates are usually set to mark periods of life. The divisions given below are not valid in all cultures.

• Juvenile The term minor is used to refer to a person who is under the age in which one legally assumes adulthood and is legally granted rights afforded to adults in society. Depending on the jurisdiction and application, this age may vary, but is usually marked at either 12, 16, 18, 20, or 21. Specifically, the status of minor is defined by the age of (via Infancy The term infant derives from the Latin word infans, meaning "unable to speak or speechless." It is typically applied to children between the ages of 1 month and 12 months; however, definitions vary between birth and 3 years of age / Childhood Childhood is the age span ranging from birth to adolescence. In developmental psychology, childhood is divided up into the developmental stages of toddlerhood , early childhood (play age), middle childhood (school age), and adolescence (post-puberty) / Adolescence Adolescence is a transitional stage of physical and mental human development generally occurring between puberty and legal adulthood (age of majority), but may also be defined as ending with the teenage stage. According to Erik Erikson's stages of human development, for example, a young adult is generally a person between the ages of 20 and 40, (Teenager Adolescence is a transitional stage of physical and mental human development generally occurring between puberty and legal adulthood (age of majority), but may also be defined as ending with the teenage stage. According to Erik Erikson's stages of human development, for example, a young adult is generally a person between the ages of 20 and 40,)): 0 – 19
• Early adulthood The term adult has at least three distinct meanings. It can indicate a biologically grown or mature person. It may also mean a plant, animal, or person who has reached full growth or alternatively is capable of reproduction, or the classification legal adult, generally determined as a person who has attained the legally fixed age of majority; as: 20 – 39
• Middle adulthood: 40 – 59
• Late adulthood: 60 –

Ages can also be divided by decade:

People from 13 to 19 years of age are also known as teens Adolescence is a transitional stage of physical and mental human development generally occuring between puberty and legal adulthood (age of majority), but may also be defined as ending with the teenage stage. According to Erik Erikson's stages of human development, for example, a young adult is generally a person between the ages of 20 and 40, or teenagers. The casual terms "twentysomething", "thirtysomething", etc. are also in use to describe people by decade or age.

Cultural variations

In some cultures (for example Serbian Serbs are a native Balkan South Slavic ethnic group. Majority of Serbs live in the Central Europe and the Balkans (Southeastern Europe), between the Balkan and Carpathian mountains, in the east, and the Adriatic sea, in the west. Serbs are a ethnicity with a large diaspora. The total world Serbian population, however, is difficult to measure) there are four ways to express age: by counting years with or without including current year. For example, it could be said about the same person that he is twenty years old or that he is in the twenty-first year of his life. In Russian the former expression is generally used, the latter one has restricted usage: it is used for age of a deceased person in obituaries and for the age of an adult when it is desired to show him/her older than he/she is. (Psychologically, a woman in her 20th year seems older than one who is 19 years old.)

Depending on cultural and personal philosophy, ageing can be seen as an undesirable phenomenon, reducing beauty and bringing one closer to death; or as an accumulation of wisdom, mark of survival, and a status worthy of respect. In some cases numerical age is important (whether good or bad), whereas others find the stage in life that one has reached (adulthood, independence, marriage, retirement, career success) to be more important.

East Asian age reckoning East Asian age reckoning is a concept and practice that originated in China and is used in East Asian cultures. Chinese culture, Japanese culture, Korean culture, Vietnamese culture, and others share this traditional way of counting a person's age. Newborns start at one year old, and each passing of a Lunar New Year, rather than the birthday, adds is different from that found in Western culture Western culture refers to cultures of European origin. Traditional Chinese culture The Culture of China is one of the world's oldest and most complex cultures. The area in which the culture is dominant covers a large geographical region in eastern Asia with customs and traditions varying greatly between towns, cities and provinces uses a different ageing method, called Xusui (虛歲) with respect to common ageing which is called Zhousui (周歲). In the Xusui method, people are born at age 1, not age 0, because conception is already considered to be the start of the life span,^{[citation needed]} and another difference is the ageing day: Xusui grows up at the Spring Festival (aka. Chinese New Year's Day), while Zhousui grows up at one's birthday.

Society

Legal

There are variations in many countries as to what age a person legally becomes an adult.

Most legal systems define a specific age for when an individual is allowed or obliged to do something. These ages include voting age A voting age is a minimum age established by law that a person must attain to be eligible to vote in a public election. The vast majority of countries in the world have established a voting age. Most governments consider that those of no age lower than the chosen threshold lack the necessary capacity to independently decide how to cast a vote. The, drinking age The legal drinking age refers to the earliest age in a country that a person is legally allowed to buy alcoholic beverages, which may be different from the age at which they may be permitted to drink alcohol, especially in the privacy of their home. The legal drinking age varies around the world in general, from countries in Europe where people, age of consent While the phrase age of consent typically does not appear in legal statutes, when used in relation to sexual activity, the age of consent is the minimum age at which a person is considered to be legally competent of consenting to sexual acts. The European Union calls it the legal age for sexual activities. It should not be confused with the age of, age of majority The age of majority is the threshold of adulthood as it is conceptualized in law. It is the chronological moment when a minor ceases to legally be considered a child and assumes control over their persons, actions, and decisions, thereby terminating the legal control and legal responsibilities of their parents or guardian over and for them. The, age of criminal responsibility The defense of infancy is a form of defense known as an excuse so that defendants falling within the definition of an "infant" are excluded from criminal liability for their actions, if at the relevant time, they had not reached an age of criminal responsibility. After reaching the initial age, there may be levels of responsibility, marriageable age, age of candidacy, and mandatory retirement age. Admission to a movie for instance, may depend on age according to a motion picture rating system. A bus fare might be discounted for the young or old.

Similarly in many countries in jurisprudence, the defence of infancy is a form of defence by which a defendant argues that, at the time a law was broken, they were not liable for their actions, and thus should not be held liable for a crime. Many courts recognise that defendants who are considered to be juveniles may avoid criminal prosecution on account of their age, and in borderline cases the age of the offender is often held to be a mitigating circumstance.

Economics and marketing

The economics of ageing are also of great importance. Children and teenagers have little money of their own, but most of it is available for buying consumer goods. They also have considerable impact on how their parents spend money.

Young adults are an even more valuable cohort. They often have an income but few responsibilities such as a mortgage or children. They do not yet have set buying habits and are more open to new products.

The young are thus the central target of marketers.^[8] Television is programmed to attract the range of 15 to 35 year olds. Mainstream movies are also built around appealing to the young.

Health care demand

Many societies in Western Europe and Japan, have ageing populations. While the effects on society are complex, there is a concern about the impact on health care demand. The large number of suggestions in the literature for specific interventions to cope with the expected increase in demand for long-term care in ageing societies can be organized under four headings: improve system performance; redesign service delivery; support informal caregivers; and shift demographic parameters.^[9]

However, the annual growth in national health spending is not mainly due to increasing demand from ageing populations, but rather has been driven by rising incomes, costly new medical technology, a shortage of health care workers and informational asymmetries between providers and patients.^[10]

Even so, it has been estimated that population ageing only explains 0.2 percentage points of the annual growth rate in medical spending of 4.3 percent since 1970. In addition, certain reforms to Medicare decreased elderly spending on home health care by 12.5 percent per year between 1996 and 2000.^[11] This would suggest that the impact of ageing populations on health care costs is not inevitable.

Impact on prisons

As of July 2007, medical costs for a typical inmate in the United States might run an agency around $33 per day, while costs for an ageing inmate could run upwards of $100. Most State DOCs report spending more than 10 percent of the annual budget on elderly care. That is expected to rise over the next 10–20 years. Some states have talked about releasing ageing inmates early.^[12]

Cognitive effects

Steady decline in many cognitive processes is seen across the lifespan, starting in one's thirties. Research has focused in particular on memory and ageing, and has found decline in many types of memory with ageing, but not in semantic memory or general knowledge such as vocabulary definitions, which typically increases or remains steady. Early studies on changes in cognition with age generally found declines in intelligence in the elderly, but studies were cross-sectional rather than longitudinal and thus results may be an artifact of cohort rather than a true example of decline. Intelligence may decline with age, though the rate may vary depending on the type, and may in fact remain steady throughout most of the lifespan, dropping suddenly only as people near the end of their lives. Individual variations in rate of cognitive decline may therefore be explained in terms of people having different lengths of life.^[3] There are changes to the brain: though neuron loss is minor after 20 years of age there is a 10% reduction each decade in the total length of the brain's myelinated axons.^[13]

Coping and well-being

Psychologists have examined coping skills in the elderly. Various factors, such as social support, religion and spirituality, active engagement with life and having an internal locus of control have been proposed as being beneficial in helping people to cope with stressful life events in later life.^[14][15][16] Social support and personal control are possibly the two most important factors that predict well-being, morbidity and mortality in adults.^[17] Other factors that may link to well-being and quality of life in the elderly include social relationships (possibly relationships with pets as well as humans), and health.^[18]

Individuals in different wings in the same retirement home have demonstrated a lower risk of mortality and higher alertness and self-rated health in the wing where residents had greater control over their environment,^[19][20] though personal control may have less impact on specific measures of health.^[16] Social control, perceptions of how much influence one has over one's social relationships, shows support as a moderator variable for the relationship between social support and perceived health in the elderly, and may positively influence coping in the elderly.^[21]

Religion

Religion has been an important factor used by the elderly in coping with the demands of later life, and appears more often than other forms of coping later in life.^[22] Religious commitment may also be associated with reduced mortality,^{[citation needed]} though religiosity is a multidimensional variable; while participation in religious activities in the sense of participation in formal and organized rituals may decline, it may become a more informal, but still important aspect of life such as through personal or private prayer.^[23]

Self-rated health

Self-ratings of health, the beliefs in one's own health as excellent, fair or poor, has been correlated with well-being and mortality in the elderly; positive ratings are linked to high well-being and reduced mortality.^[24][25] Various reasons have been proposed for this association; people who are objectively healthy may naturally rate their health better than that of their ill counterparts, though this link has been observed even in studies which have controlled for socioeconomic status, psychological functioning and health status.^[26] This finding is generally stronger for men than women,^[25] though the pattern between genders is not universal across all studies, and some results suggest sex-based differences only appear in certain age groups, for certain causes of mortality and within a specific sub-set of self-ratings of health.^[26]

Retirement

Retirement, a common transition faced by the elderly, may have both positive and negative consequences.^[27]

Societal impact

Of the roughly 150,000 people who die each day across the globe, about two thirds — 100,000 per day — die of age-related causes.^[2] In industrialized nations, the proportion is much higher, reaching 90%.^[2]

Societal ageing refers to the demographic ageing of populations and societies.^[28] Cultural differences in attitudes to ageing have been studied.^{[citation needed]}

Emotional improvement

Given the physical and cognitive declines seen in ageing, a surprising finding is that emotional experience improves with age.^{[citation needed]} Older adults are better at regulating their emotions and experience negative affect less frequently than younger adults and show a positivity effect in their attention and memory.^{[citation needed]} The emotional improvements show up in longitudinal studies^[specify] as well as in cross-sectional studies^[specify] and so cannot be entirely due to only the happier individuals surviving.

Successful ageing

The concept of successful ageing can be traced back to the 1950s, and popularised in the 1980s. Previous research into ageing exaggerated the extent to which health disabilities, such as diabetes or osteoporosis, could be attributed exclusively to age, and research in gerontology exaggerated the homogeneity of samples of elderly people.^[29][30]

Successful ageing consists of three components:^[31]

1. Low probability of disease or disability;
2. High cognitive and physical function capacity;
3. Active engagement with life.

A greater number of people self-report successful ageing than those that strictly meet these criteria.^[29]

Successful ageing may be viewed an interdisciplinary concept, spanning both psychology and sociology, where it is seen as the transaction between society and individuals across the life span with specific focus on the later years of life.^[32] The terms "healthy ageing"^[29] "optimal ageing" have been proposed as alternatives to successful ageing.

Six suggested dimensions of successful ageing include:^[16]

1. No physical disability over the age of 75 as rated by a physician;
2. Good subjective health assessment (i.e. good self-ratings of one's health);
3. Length of undisabled life;
4. Good mental health;
5. Objective social support;
6. Self-rated life satisfaction in eight domains, namely marriage, income-related work, children, friendship and social contacts, hobbies, community service activities, religion and recreation/sports.

Theories

Biological theories

At present, the biological basis of ageing is unknown. Most scientists agree that substantial variability exists in the rates of ageing across different species, and that this to a large extent is genetically based. In model organisms and laboratory settings, researchers have been able to demonstrate that selected alterations in specific genes can extend lifespan (quite substantially in nematodes, less so in fruit flies, and even less in mice). Nevertheless, even in the relatively simple organisms, the mechanism of ageing remain to be elucidated. Because the lifespan of even the simple lab mouse is around 3 years, very few experiments directly test specific ageing theories (most of the evidence for the ones listed below is correlative).

The US National Institute on Aging currently funds an intervention testing program, whereby investigators nominate compounds (based on specific molecular ageing theories) to have evaluated with respect to their effects on lifespan and age-related biomarkers in outbred mice.^[33] Previous age-related testing in mammals has proved largely irreproducible, because of small numbers of animals, and lax mouse husbandry conditions. The intervention testing program aims to address this by conducting parallel experiments at three internationally recognized mouse ageing-centres, the Barshop Institute at UTHSCSA, the University of Michigan at Ann Arbor and the Jackson Laboratory.

Telomere Theory

Telomeres (structures at the ends of chromosomes) have experimentally been shown to shorten with each successive cell division. Shortened telomeres activate a mechanism that prevents further cell multiplication. This may be an important mechanism of ageing in tissues like bone marrow and the arterial lining where active cell division is necessary. Importantly though, mice lacking telomerase enzyme do not show a dramatically reduced lifespan, as the simplest version of this theory would predict.

Reproductive-Cell Cycle Theory

The idea that ageing is regulated by reproductive hormones that act in an antagonistic pleiotropic manner via cell cycle signalling, promoting growth and development early in life in order to achieve reproduction, but later in life, in a futile attempt to maintain reproduction, become dysregulated and drive senescence (dyosis).

Wear-and-Tear Theory

The very general idea that changes associated with ageing are the result of chance damage that accumulates over time.

Somatic Mutation Theory

The biological theory that ageing results from damage to the genetic integrity of the body’s cells.

Error Accumulation Theory

The idea that ageing results from chance events that escape proof reading mechanisms, which gradually damages the genetic code.

The Viral Theory of Aging

Known causes of cancer (radiation, chemical and viral) account for about 30% of the total cancer burden and for about 30% of the total DNA damage. DNA damage causes the cells to stop dividing or induce apoptosis. DNA damage is thought to be the common pathway causing both cancer and aging. It seems unlikely that the estimates of the DNA damage due to radiation and chemical causes has been significantly underestimated. Viral infection would appear to be the most likely cause of the other 70% of DNA damage especially in cells that are not exposed to smoking and sun light.^[34]

Evolutionary Theories

These are speculative and they do not provide readily testable methods of confirmation.

Accumulative-Waste Theory

The biological theory of ageing that points to a buildup of cells of waste products that presumably interferes with metabolism.

Autoimmune Theory

The idea that ageing results from an increase in autoantibodies that attack the body's tissues. A number of diseases associated with ageing, such as atrophic gastritis and Hashimoto's thyroiditis, are probably autoimmune in this way. While inflammation is very much evident in old mammals, even SCID mice in SPF colonies still senescence.

Ageing-Clock Theory

The theory that ageing results from a preprogrammed sequence, as in a clock, built into the operation of the nervous or endocrine system of the body. In rapidly dividing cells the shortening of the telomeres would provide just such a clock. This idea is in direct contradiction with the evolutionary based theory of ageing.

Cross-Linkage Theory

The idea that ageing results from accumulation of cross-linked compounds that interfere with normal cell function.

Free-Radical Theory

The idea that free radicals (unstable and highly reactive organic molecules, also named reactive oxygen species or oxidative stress) create damage that gives rise to symptoms we recognize as ageing.

Reliability theory of ageing and longevity

A general theory about systems failure. It allows researchers to predict the age-related failure kinetics for a system of given architecture (reliability structure) and given reliability of its components. Reliability theory predicts that even those systems that are entirely composed of non-ageing elements (with a constant failure rate) will nevertheless deteriorate (fail more often) with age, if these systems are redundant in irreplaceable elements. Ageing, therefore, is a direct consequence of systems redundancy. Reliability theory also predicts the late-life mortality deceleration with subsequent levelling-off, as well as the late-life mortality plateaus, as an inevitable consequence of redundancy exhaustion at extreme old ages. The theory explains why mortality rates increase exponentially with age (the Gompertz law) in many species, by taking into account the initial flaws (defects) in newly formed systems. It also explains why organisms "prefer" to die according to the Gompertz law, while technical devices usually fail according to the Weibull (power) law. Reliability theory allows to specify conditions when organisms die according to the Weibull distribution: organisms should be relatively free of initial flaws and defects. The theory makes it possible to find a general failure law applicable to all adult and extreme old ages, where the Gompertz and the Weibull laws are just special cases of this more general failure law. The theory explains why relative differences in mortality rates of compared populations (within a given species) vanish with age (compensation law of mortality), and mortality convergence is observed due to the exhaustion of initial differences in redundancy levels.

Mitohormesis

It has been known since the 1930s that restricting calories while maintaining adequate amounts of other nutrients can extend lifespan in laboratory animals. Recently, Michael Ristow's group has provided evidence for the theory that this effect is due to increased formation of free radicals within the mitochondria causing a secondary induction of increased antioxidant defence capacity.^[35]

Misrepair-Accumulation Theory: This very recent novel theory by Wang et al.^[36] suggests that ageing is the result of the accumulation of "Misrepair". Important in this theory is to distinguish among "damage" which means a newly emerging defect BEFORE any reparation has taken place, and "Misrepair" which describes the remaining defective structure AFTER (incorrect) repair. The key points in this theory are:

• There is no original damage left unrepaired in a living being. If damage was left unrepaired a life threatening condition (such as bleeding, infection, or organ failure) would develop.
• Misrepair, the repair with less accuracy, does not happen accidentally. It is a necessary measure of the reparation system to achieve sufficiently quick reparation in situations of serious or repeated damage, to maintain the integrity and basic function of a structure, which is important for the survival of the living being.
• Hence the appearance of Misrepair increases the chance for the survival of individual, by which the individual can live at least up to the reproduction age, which is critically important for the survival of species. Therefore the Misrepair mechanism was selected by nature due to its evolutionary advantage.
• However, since Misrepair as a defective structure is invisible for the reparation system, it accumulates with time and causes gradually the disorganization of a structure (tissue, cell, or molecule); this is the actual source of ageing.
• Ageing hence is the side-effect for survival, but important for species survival. Thus Misrepair might represent the mechanism by which organisms are not programmed to die but to survive (as long as possible), and ageing is just the price to be paid.

Non-biological theories

Disengagement Theory

This is the idea that separation of older people from active roles in society is normal and appropriate, and benefits both society and older individuals. Disengagement theory, first proposed by Cumming and Henry, has received considerable attention in gerontology, but has been much criticised.^[3] The original data on which Cumming and Henry based the theory were from a rather small sample of older adults in Kansas City, and from this select sample Cumming and Henry then took disengagement to be a universal theory.^[37] There are research data suggesting that the elderly who do become detached from society as those were initially reclusive individuals, and such disengagement is not purely a response to ageing.^[3]

Activity Theory

In contrast to disengagement theory, this theory implies that the more active elderly people are, the more likely they are to be satisfied with life. The view that elderly adults should maintain well-being by keeping active has had a considerable history, and since 1972, this has become to be known as activity theory.^[37] However, this theory may be just as inappropriate as disengagement for some people as the current paradigm on the psychology of ageing is that both disengagement theory and activity theory may be optimal for certain people in old age, depending on both circumstances and personality traits of the individual concerned.^[3] There are also data which query whether, as activity theory implies, greater social activity is linked with well-being in adulthood.^[37]

Selectivity Theory

mediates between Activity and Disengagement Theory, which suggests that it may benefit older people to become more active in some aspects of their lives, more disengaged in others.^[37]

Continuity Theory

The view that in ageing people are inclined to maintain, as much as they can, the same habits, personalities, and styles of life that they have developed in earlier years. Continuity theory is Atchley's theory that individuals, in later life, make adaptations to enable them to gain a sense of continuity between the past and the present, and the theory implies that this sense of continuity helps to contribute to well-being in later life.^[18] Disengagement theory, activity theory and continuity theory are social theories about ageing, though all may be products of their era rather than a valid, universal theory.

Prevention and reversal

Several drugs and food supplements have been shown to retard or reverse the biological effects of ageing in animal models; none has yet been proven to do so in humans.

Resveratrol, a chemical found in red grapes, has been shown to extend the lifespan of yeast by 60%, worms and flies by 30% and one species of fish by almost 60%. Small doses of heavy water increase fruit-fly lifespan by 30%, but large doses are toxic to complex organisms.

In 2002, a team led by Professor Bruce Ames at UC Berkeley discovered that feeding aged rats a combination of acetyl-L-carnitine and alpha-lipoic acid (both substances are already approved for human use and sold in health food stores) produced a rejuvenating effect.^[38] Ames said, "With these two supplements together, these old rats got up and did the macarena. The brain looks better, they are full of energy - everything we looked at looks like a young animal." UC Berkeley has patented the use of these supplements in combination and a company, Juvenon, has been established to market the treatment.

In 2007, researchers at the Salk Institute for Biological Studies, identified a critical gene in nematode worms that specifically links eating fewer calories with living longer. Professor Andrew Dillin and colleagues showed that the gene pha-4 regulates the longevity response to calorie restriction.^[39] In the same year Dr Howard Chang of the Stanford University School of Medicine was able to rejuvenate the skin of two-year-old mice to resemble that of newborns by blocking the activity of the gene NF-kappa-B.^[40]

In 2008, a team at the Spanish National Cancer Research Center genetically engineered mice to produce ten times the normal level of the telomerase enzyme.^[41] The mice lived 26% longer than normal.^[42] The same year a team led by Professor Michael O Thorner at the ^[43] University of Virginia discovered that the drug MK-677 restored 20% of muscle mass lost due to ageing in humans aged 60 to 81. The subjects' growth hormone and insulin-like growth factor 1 (IGF-1) levels increased to that typical of healthy young adults.^[44]

In 2009, a drug called rapamycin, discovered in the 1970s in the soil of Easter Island in the South Pacific, was found to extend the life expectancy of 20-month-old mice by up to 38%.^[45] Rapamycin is generally used to suppress the immune system and prevent the rejection of transplanted organs. Dr Arian Richardson of the Barshop Institute said, "I never thought we would find an anti-ageing pill in my lifetime; however, rapamycin shows a great deal of promise to do just that." Professor Randy Strong of the University of Texas Health Science Center at San Antonio said, "We believe this is the first convincing evidence that the ageing process can be slowed and lifespan can be extended by a drug therapy starting at an advanced age."

Also in 2009, the British Journal of Nutrition reported a study at Tufts University in Boston which showed that brain function and motor skills in aged rats could be improved by adding walnuts to their diet. The human equivalent would be to eat seven to nine walnuts per day.^[46]

In September 2009, researchers at UC Berkeley discovered they could restore youthful repair capability to muscle tissue taken from men aged 68 to 74 by in vitro treatment with mitogen-activated protein kinase.^[47] This protein was found to be essential for the production of the stem cells necessary to repair muscle after exercise and is present at reduced levels in aged individuals.

Measure of age

The age of an adult human is commonly measured in whole years since the day of birth. Fractional years, months or even weeks may be used to describe the age of children and infants for finer resolution. The time of day the birth occurred is not commonly considered.

The measure of age has historically varied from this approach in some cultures. For example, in China, Korea, Japan and Vietnam, children were considered to be one year old at the moment of birth and two years old on the following New Year's Day.^[48] Thus, a child could be considered two years old several days after birth. In parts of Tibet, age is counted from conception i.e. one is 9 months old when one is born.^[49]

Age in prenatal development is normally measured in gestational age, taking the last menstruation of the woman as a point of beginning. Alternatively, fertilisation age, beginning from fertilisation can be taken.

See also

• Ageing brain
• Ageing of Europe
• Biodemography
• Biological immortality
• Gerontology
• Life expectancy
• List of life extension-related topics
• Longevity
• Memory and ageing
• Population ageing
• Retirement
• Senescence

Notes

1. ^ Bowen RL, Atwood CS (2004). "Living and dying for sex. A theory of aging based on the modulation of cell cycle signaling by reproductive hormones.". Gerontology 50 (5): 265–90. doi:10.1159/000079125. PMID 15331856.
2. ^ ^{a b c} Aubrey D.N.J, de Grey (2007). "Life Span Extension Research and Public Debate: Societal Considerations" (PDF). Studies in Ethics, Law, and Technology 1 (1, Article 5). doi:10.2202/1941-6008.1011. http://www.mfoundation.org/files/sens/ENHANCE-PP.pdf. Retrieved March 20, 2009.
3. ^ ^{a b c d e} Stuart-Hamilton, Ian (2006). The Psychology of Ageing: An Introduction. London: Jessica Kingsley Publishers. ISBN 1-84310-426-1.
4. ^ Physiological Basis of Ageing and Geriatrics, By Paola S. Timiras, p.26, Published 2003 Informa Health Care, ISBN 0849309484
5. ^ Jacob Silverman. "Is there a 400 pound lobster out there?". howstuffworks. http://animals.howstuffworks.com/marine-life/400-pound-lobster.htm/printable.
6. ^ David Foster Wallace (2005). Consider the Lobster and Other Essays. Little, Brown & Company. ISBN 0-31-615611-6.
7. ^ Hanahan D, Weinberg RA (2000). "The hallmarks of cancer". Cell 100 (1): 57–70. doi:10.1016/S0092-8674(00)81683-9. PMID 10647931.
8. ^ Krulwich, Robert (2006). . All Things Considered. NPR. . Retrieved 2006-08-22.
9. ^ Saltman, R.B.; Dubois, H.F.W.; Chawla, M. (2006). "The Impact Of Ageing On Long-term Care In Europe And Some Potential Policy Responses". International Journal of Health Services 36 (4): 719–746. doi:10.2190/AUL1-4LAM-4VNB-3YH0. PMID 17175843. http://baywood.metapress.com/index/AUL14LAM4VNB3YH0.pdf. Retrieved 2008-02-11.
10. ^ Rheinhardt, Uwe E. (2003). "Does The Ageing Of The Population Really Drive The Demand For Health Care?". Health Affairs 22 (6): 27–39. doi:10.1377/hlthaff.22.6.27. PMID 14649430. http://healthaff.highwire.org/cgi/reprint/22/6/27.pdf. Retrieved 2008-04-17.
11. ^ Meara, Ellen; White, Chapin; Cutler, David M. (2004). "Trends in medical spending on by age, 1963-2000". Health Affairs 23 (4): 176–183. doi:10.1377/hlthaff.23.4.176. PMID 15318578. http://content.healthaffairs.org/cgi/reprint/23/4/176.pdf. Retrieved 2008-04-10.
12. ^ Aday, Ronald H. (2003). Aging Prisoners: Crisis in American Corrections. Praeger. ISBN 0275971236.
13. ^ Marner L, Nyengaard JR, Tang Y, Pakkenberg B. (2003). Marked loss of myelinated nerve fibers in the human brain with age. J Comp Neurol. 462(2):144-52. PubMed
14. ^ Schulz R, Heckhausen J (1996). "A life span model of successful ageing". Am Psychol 51 (7): 702–14. doi:10.1037/0003-066X.51.7.702. PMID 8694390.
15. ^ Windsor TD, Anstey KJ, Butterworth P, Luszcz MA, Andrews GR (2007). "The role of perceived control in explaining depressive symptoms associated with driving cessation in a longitudinal study". Gerontologist 47 (2): 215–23. PMID 17440126.
16. ^ ^{a b c} Diane F. Gilmer; Aldwin, Carolyn M. (2003). Health, illness, and optimal ageing: biological and psychosocial perspectives. Thousand Oaks: Sage Publications. ISBN 0-7619-2259-8.
17. ^ Smith GC, Kohn SJ, Savage-Stevens SE, Finch JJ, Ingate R, Lim YO (2000). "The effects of interpersonal and personal agency on perceived control and psychological well-being in adulthood". Gerontologist 40 (4): 458–68. PMID 10961035.
18. ^ ^{a b} Bowling, Ann (2005). Ageing well: quality of life in old age. [Milton Keynes]: Open University Press. ISBN 0335215092.
19. ^ Langer EJ, Rodin J (1976). "The effects of choice and enhanced personal responsibility for the aged: a field experiment in an institutional setting". J Pers Soc Psychol 34 (2): 191–8. doi:10.1037/0022-3514.34.2.191. PMID 1011073.
20. ^ Rodin J, Langer EJ (1977). "Long-term effects of a control-relevant intervention with the institutionalized aged". J Pers Soc Psychol 35 (12): 897–902. doi:10.1037/0022-3514.35.12.897. PMID 592095.
21. ^ Bisconti, T.L.; Bergeman, CS (1 February 1999). "Perceived social control as a mediator of the relationships among social support, psychological well-being, and perceived health". The Gerontologist 39 (1): 94–103. PMID 10028775. http://gerontologist.gerontologyjournals.org/cgi/content/abstract/39/1/94. Retrieved 2008-02-11.
22. ^ McFadden, S (2005). Gerontology and the Psychology of Religion ; inPark, Crystal L.; Raymond F. Paloutzian (2005). Handbook of the Psychology of Religion and Spirituality. New York: The Guilford Press. ISBN 1-57230-922-9.
23. ^ Mindel CH, Vaughan CE (1978). "A multidimensional approach to religiosity and disengagement". J Gerontol 33 (1): 103–8. PMID 618958.
24. ^ Idler, E.L. (1 June 2003). "Discussion: Gender Differences in Self-Rated Health, in Mortality, and in the Relationship Between the Two". The Gerontologist 43 (3): 372–375. http://gerontologist.gerontologyjournals.org/cgi/content/full/43/3/372. Retrieved 2008-02-11.
25. ^ ^{a b} Deeg, D.J.H.; Bath, P.A. (1 June 2003). "Self-Rated Health, Gender, and Mortality in Older Persons: Introduction to a Special Section". The Gerontologist 43 (3): 369–371. PMID 12810900. http://gerontologist.gerontologyjournals.org/cgi/content/full/43/3/369. Retrieved 2008-02-11.
26. ^ ^{a b} Benyamini, Y.; Blumstein, T.; Lusky, A.; Modan, B. (1 June 2003). "Gender Differences in the Self-Rated Health-Mortality Association: Is It Poor Self-Rated Health That Predicts Mortality or Excellent Self-Rated Health That Predicts Survival?". The Gerontologist 43 (3): 396–405. PMID 12810904. http://gerontologist.gerontologyjournals.org/cgi/content/abstract/43/3/396. Retrieved 2008-02-11.
27. ^ Panek, Paul E.; Hayslip, Bert (1989). Adult development and ageing. San Francisco: Harper & Row. ISBN 0060450126.
28. ^ Sarah Harper, 2006, Ageing Societies: Myths, Challenges and Opportunities.
29. ^ ^{a b c} Strawbridge WJ, Wallhagen MI, Cohen RD (2002). "Successful ageing and well-being: self-rated compared with Rowe and Kahn". Gerontologist 42 (6): 727–33. PMID 12451153.
30. ^ Rowe JW, Kahn RL (1987). "Human ageing: usual and successful". Science 237 (4811): 143–9. doi:10.1126/science.3299702. PMID 3299702.
31. ^ Rowe JW, Kahn RL (1997). "Successful ageing". Gerontologist 37 (4): 433–40. PMID 9279031.
32. ^ Fentleman, DL; Smith, J & Peterson, J (1990). Successful ageing in a postretirement society ; in Baltes, Margret M.; Baltes, Paul B. (1990). Successful ageing: perspectives from the behavioral sciences. Cambridge, UK: Cambridge University Press. ISBN 052143582X.
33. ^ An Ageing Interventions Testing Program: study design and interim report. Ageing Cell. 2007 Aug;6(4):565-75.
34. ^ DNA Damage as the Primary Cause of Aging, by Helen L. Gensler and Harris Bernstein quarterly review of biology volume 56 page 279 © 1981 The University of Chicago Press
35. ^ Schulz TJ, Zarse K, Voigt A, Urban N, Birringer M, Ristow M (2007). "Glucose restriction extends Caenorhabditis elegans life span by inducing mitochondrial respiration and increasing oxidative stress". Cell Metab. 6 (4): 280–93. doi:10.1016/j.cmet.2007.08.011. PMID 17908557.
36. ^ Wang J, Michelitsch T, Wunderlin A, Mahadeva R. "Ageing as a Consequence of Misrepair -- a Novel Theory of Ageing". ArXiv:0904.0575. http://arxiv.org/abs/0904.0575
37. ^ ^{a b c d} Willis, Sherry L. (1996). Adult development and ageing. New York, NY: HarperCollins College Publishers. ISBN 0673994023.
38. ^ Dietary Supplements Make Old Rats Youthful, May Rejuvenate Aging Humans
39. ^ http://www.salk.edu/faculty/dillin.html
40. ^ Gene Tweak Reverses Aging in Mouse Skin Cells
41. ^ Spanish National Cancer Research Center
42. ^ Telomere enzyme a likely key to longevity
43. ^ http://www.healthsystem.virginia.edu
44. ^ Annals of Internal Medicine, Nov 4, 2008
45. ^ Tests raise life extension hopes
46. ^ Adding Walnuts To Good Diet May Help Older People Improve Motor And Behavioral Skills
47. ^ Scientists discover clues to what makes human muscle age
48. ^ Segal, Ethan. "Windows on Asia: Rethinking Time, Age, and the Calendar". http://asia.msu.edu/Study%20Units/unit1M2L3.htm. Retrieved 2008-06-29.
49. ^ Maddison, Angus (2006). The World Economy. Paris: OECD. pp. 31. ISBN 9264022619. http://books.google.com/?id=DF-N_lXjlL8C&pg=PA31&lpg=PA31&dq=japanese+%221+year+old%22+birth. Retrieved 2008-06-28.

References

• Bass, S.A. (2006). Gerontological Theory: The Search for the Holy Grail. The Gerontologist, 46, 139-144.
• Bath, P.A. (2003). Differences between older men and woman in the Self-Rated Health/ Mortality Relationship. The Gerontologist, 43 387-94
• Charles, S.T., Reynolds, C.A., & Gatz, M. (2001). Age-related differences and change in positive and negative affect over 23 years. Journal of Personality and Social Psychology, 80, 136-151.
• Mather, M., & Carstensen, L. L. (2005). Ageing and motivated cognition: The positivity effect in attention and memory. Trends in Cognitive Sciences 9, 496-502. PDF
• Masoro E.J. & Austad S.N.. (eds.): Handbook of the Biology of Ageing, Sixth Edition. Academic Press. San Diego, CA, USA, 2006. ISBN 0-12-088387-2
• Moody, Harry R. Ageing: Concepts and Controversies. 5th ed. California: Pine Forge Press, 2006.
• Zacks, R.T., Hasher, L., & Li, K.Z.H. (2000). Human memory. In F.I.M. Craik & T.A. Salthouse (Eds.), The Handbook of Ageing and Cognition (pp. 293–357). Mahwah, NJ: Erlbaum.
• A small-cell lung cancer genome with complex signatures of tobacco exposure Nature 463, 184-190 (14 January 2010)

External links

• American Psychological Association's Office on Aging

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