The gradual changes in the structure and function of humans and animals that occur with the passage of time, that do not result from disease or other gross accidents, and that eventually lead to the increased probability of death as the person or animal grows older. It does not apply to microorganisms.

Source: BIOLOGY ONLINE: http://www.biology-online.org/dictionary/Aging

Aging, the process of growing old, is defined as the gradual biological impairment of normal function, probably as a result of changes made to cells (mitotic cells, such as fibroblasts and post-mitotic cells, such as neurons) and structural components (such as bone and muscle). These changes would consequently have a direct impact on the functional ability of organs (such as the heart, kidney and lungs), biological systems (such as the nervous, digestive and reproductive system) and ultimately the organism as a whole.

The following is a description setting out five criteria’s for aging, as proposed by Strehler (1962).

• Cumulative: Effects of aging increase with time.
• Universal: All members of a species display signs of aging.
• Progressive: Aging is a series of gradual changes.
• Intrinsic: Changes would take place even in a “perfect” environment.
• Deleterious: Changes which occur compromise normal biological functions.

Like aging, disease is also defined as an impairment of normal function within a living organism. Since the aging process leads to biological impairment, it would not therefore be a surprise if some of these age-associated changes manifest themselves as disease. Evidence providing a link between aging mechanisms and age-related disease development/progression is gradually increasing (discussed at a later date).

Source: AGING RESEARCH: www.aging-research.blogspot.com/2007/12/definition-of-normal-aging.html

Human Life Span  The Aging Process  Aging and Hormones  Extending Life