Nature brings life from conception to the peak of reproductive capacity. After that, there is no genetic plan for maintenance: Life runs on reserve power (see Chapter 65). Aging is not a disease, although the incidence of disease increases with age, and disease can make people age more quickly.

Stochastic theories propose that aging events are random, cumulative, and microscopic. For example, random exposure to radiation can induce mutations in DNA, causing cells to malfunction. The slow accumulation of damage to cell membranes by free radicals (the by-products of oxygen metabolism) is another stochastic theory. The free radical theory is related to the cross-link, or connective tissue, theory. It proposes that chemical reactions bind molecular structures that work best when separate, such as proteins, nucleic acid, and collagen. These structures change the composition of soft tissue, such as skin, muscles, and blood vessels, thereby compromising their effectiveness. Nonstochastic theories propose that aging is genetically programmed into the lives of cells. For example, a genetic timetable shuts down the ovaries, causing menopause.