|
Senescence In biology, senescence is the state or process of ageing. Cellular senescence is a phenomenon where isolated cells demonstrate a limited ability to divide in culture. Organismal senescence is the ageing of organisms. Ageing is believed to have evolved because of the increasingly smaller probability of an organism still being alive at older age, due to predation and accidents, both of which may be entirely random and age-invariant. It is thought that strategies which result in a higher reproductive rate at a young age, but shorter overall lifespan, result in a higher lifetime reproductive success and are therefore favoured by natural selection. Organismal ageing is generally characterized by the declining ability to respond to stress, increasing homeostatic imbalance and increased risk of disease. Because of this, death is the ultimate consequence of ageing. Some researchers are treating ageing as a "disease" in gerontology (specifically biogerontologists), although this view is controversial. This view is, however, backed up by Nobel-prize winning research. In scientific circles, the controversy of treating ageing as a disease is decreasing. That is, as genes that have an effect on ageing are discovered, ageing is increasingly being regarded in a similar fashion to other genetic conditions, including genetic diseases. For example, the sirtuin family of genes have been shown to have a significant effect on the lifespan of yeast and nematodes. Similarly, the discovery of genes for Tay-Sachs disease and insulin production reveal that many human conditions are genetically based. At the cell level, it has been shown that cells can be immortalized by the introduction of an additional gene for telomerase. In cell biology, there are links to deceleration of ageing by the presence of nutritionally derived polyphenol antioxidants. Dividing the lifespan A human life is often arbitrarily divided into various ages. Because biological changes are slow moving and vary from person to person arbitrary dates are usually set to mark periods of human life. In some cultures the divisions given below are quite varied. In the USA, adulthood legally begins at the age of eighteen or nineteen, while old age is considered to begin at the age of legal retirement (approximately 65). Ages can also be divided by decade: See also Seven ages of man for an older system of dividing the human life. In some cultures (for example Serbian and Russian) there are two 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 twenty-first year of his life. Legal There are variations in many countries as to what age a person legally becomes an adult. In the United States there are issues such as voting age, drinking age, age of consent, age of majority, age of criminal responsibility, marriageable age, age where one can hold public office, and mandatory retirement age. Admission to a movie for instance, may depend on age according to a motion picture rating system. Similarly in the United States 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 recognize that defendants, which are considered to be juveniles, may avoid criminal prosecution on account of their age. Economics and marketing The economics of ageing are also of great import. 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 jobs with 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. Television is programmed to attract the 15 to 35 years olds. Movies are also built around appealing to the young. The middle aged and the old are less likely to buy things and are traditionally viewed as being set in their buying habits and not nearly as open to marketing. Older people tend to be much wealthier and to save a much higher percentage of their income. Some tax systems attempt to address these differences in age spending habits such as the concept of a lifetime income tax. Global trends There have been small changes in age distribution between 1990 and 2000. The percentage of the population that is older increased slightly between 1990 and 2000 from 9% to almost 10%. The increase was larger within more developed countries, from 17.7% to 19.4%. The percentage of the population that is older is almost three times as high in more developed countries (19.4%) as it is in less developed countries (7.7%). Cultural variations Considerable numbers of cultures have less of a problem with age compared with what has been described above, and it is seen as an important status to reach stages in life, rather than defined numerical ages. Advanced age is given more respect and status. Traditional Chinese culture use different ageing method, called Xusui (虛歲) with respect to common ageing which called Zhousui (周歲). In the Xusui method, people are born at age 1, not age 0. See also East Asian age reckoning for more information. Cognitive Steady decline in many cognitive processes are 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. Emotional Given the physical and cognitive declines seen in ageing, a surprising finding is that emotional experience improves with age. 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. The emotional improvements show up in longitudinal studies as well as in cross-sectional studies, and so cannot be entirely due to only the happier individuals surviving. Successful ageing The concept of "successful ageing", as Strawbridge et al. (2002), have pointed out, can be traced back to the 1950s, but was popularised in an article by Rowe and Kahn (1987). These authors believed that former research into ageing had exaggerated the extent to which health disabilities, such as diabetes or osteoporosis, could be attributed exclusively to age, and also criticised former research in gerontology for exaggerating the homogeneity of samples of elderly people. In a subsequent publication, Rowe and Kahn (1997) criticise earlier work for making what, to them, is an over-simplistic distinction between pathologic and non-pathologic ageing, and distinguish between "normal ageing" (marked by high risk of illness), and "successful ageing" (marked by low risk of disability and high cognitive and physical functioning). They define "successful ageing" more specifically as consisting of three components: 1. Low probability of disease or disability; 2. high cognitive and physical function capacity; 3. active engagement with life. Criticisms of Rowe and Kahn's work has been noted by Strawbridge et al. (2002), who note that more liberal definitions of "successful ageing" than those proposed by Rowe and Kahn result in greater percentages of elderly adults reaching successful ageing, and that self-reported successful ageing suggests a greater number of elderly people reach successful ageing than does an operational measure based on Rowe and Kahn's conceptualisations. Indeed, Strawbridge et al. (2002) note that the term "successful ageing",insofar as it implies competitiveness and that "normal ageing" is a failure, is itself problematic, and review alternative terms (such as "healthy ageing") that have been proposed. See also Articles Lists Notes | ||||||||||
|
| |||||||||||
 |
|
| |