Aging is a progressive, predictable process in which gradual changes not caused by disease or accident occur over time. Aging affects both the body and the mind. Some age-related changes start as early as the 20s; others may not appear until people are in their 70s. Although aging is inevitable, people age at different rates. Age-related changes eventually lead to the increased probability of death as people grow older.

Life expectancy-how long people are expected to live-has risen dramatically since 1900, when it was 47 years. Today, the average life expectancy is 76 years, an increase of about 110 days each year since 1900. Women live longer than men. People today have a much more positive attitude toward aging than earlier generations did.

The aging process involves the steady decline of organ functioning and the regulation of the body systems. Some of the age-related biological changes are predictable. Aging and disease are related...