Ageing

Ageing or aging is the process of becoming older. This article focuses on the social, cultural, and economic effects of ageing. The biology of ageing is treated in detail in senescence. 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, but wholly arbitrarily, measured in years and a person's birthday is often an important event.

There is often considerable social pressure in many societies to sustain denial of the ageing process. Considerable energy, money and time is expended to hide signs of ageing, especially among women. This may involve dyeing hair, elaborate make-up, or even cosmetic surgery. Among the young however there is often a desire to seem older to gain more responsibility and respect.

The issues of an ageing population in which the average age of a society is increasing is an important issue in many nations of the world. The societal effects of age are great. 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 thus have comparatively more political influence.

Senescence: the biology of ageing
Main article: 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.

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. 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.

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 and other countries, adulthood legally begins at the age of eighteen or nineteen, while old age is considered to begin at age sixty-five.
 * Infancy
 * Toddler
 * Childhood
 * Preteen
 * Adolescence
 * Young adult
 * Adulthood
 * Middle age
 * Old age

Ages can also be divided by decade:
 * Quadragenarian: someone between 40 and 49 years of age
 * Quinquagenarian: someone between 50 and 59 years of age
 * Sexagenarian: someone between 60 and 69 years of age
 * Septuagenarian: someone between 70 and 79 years of age
 * Octogenarian: someone between 80 and 89 years of age
 * Nonagenarian: someone between 90 and 99 years of age
 * Centenarian: someone between 100 and 109 years of age
 * Supercentenarian: someone over 110 years of age

See also Seven ages of man for an older system of dividing the human life.

Age and the law
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 of ageing
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 spent their 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.

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.

Global ageing trends
There have been small changes in age distribution between 1990 and 2000. The percent of 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 percent of 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%).

Reference

 * Global Social Change Reports One report describes global trends in ageing.