Aging is, simply put, the act of getting older. Aging is part of the natural life cycle of an organism. From birth, through maturation, and eventually to death, aging is the element that ties all segments of life together.

Life Span and the Aging Process

How long an organism lives is called its life span. In 1998, the average life span for a human, worldwide, was sixty-six years. However, life span is a complex trait, meaning that many factors, including family history, lifestyle, disease, and residence in a developed nation, determine how long an individual's life will be. The average life span in a particular population changes as these factors change. For example, the average life span in the United States in 1900 was forty-nine; in 1998 it was seventy-seven.

This increase was likely due to several factors, but perhaps the most important was the...