Old age consists of ages nearing or surpassing the average life span of human beings, and thus the end of the human life cycle. Euphemisms and terms for older people include seniors — chiefly an American usage — or elderly. As occurs with almost any definable group of humanity, some people will hold a prejudice against others; in this case, against older people. This is one form of ageism.
Older people have limited regenerative abilities and are more prone to disease, syndromes, and sickness than other adults. For the biology of ageing, see Senescence. The medical study of the aging process is gerontology, and the study of diseases that afflict the elderly is geriatrics.

Definition

The boundary between middle age and old age cannot be defined exactly because it does not have the same meaning in all societies. In many parts of the world, people are considered old because of certain changes in their activities or social roles. Examples: people may be considered old when they become grandparents, or when they begin to do less or different work — retirement. In North America and Europe, people are often considered old if they have lived a certain number of years.

Many Americans think of 65 as the beginning of old age because United States workers become eligible at this time to retire with full Social Security benefits at age 65. People in the 65-and-over age group are often called senior citizens. In 2003, the age at which an American citizen becomes eligible for full Social Security benefits began to increase gradually until it reaches 67 in 2027.

Physical manifestations

Old age can cause wrinkles and liver spots on the skin, change of hair color to grey or white or loss of hair (or both), lessened hearing and sight abilities, loss of reaction time and agility or reduced ability to think or recall memories.

Demographic changes

Worldwide, the number of people 65 or older is increasing faster than ever before. Most of this increase is occurring in developed countries. In the United States the percentage of people 65 or older increased from 4 percent in 1900 to about 13 percent in the late 1990s. In 1900, only about 3 million of the nation's citizens had reached 65. By 1998, the number of senior citizens had increased to about 34 million. Population experts estimate that more than 50 million Americans — about 17 percent of the population — will be 65 or older in 2020. The number of old people is growing around the world chiefly because more children reach adulthood.

Life expectancy

In most parts of the world, women live, on average, longer than men. In the United States in the late 1990s, life expectancy at birth was 80 years for women and 77 years for men. American women who were age 65 in the late 1990s could expect to live about 19 additional years. Men who were 65 could expect to live about 16 additional years.

Source:

http://en.wikipedia.org/wiki/Old_age