Aging: phenomena and theories.

Aging is the accumulation of diverse adverse changes that increase the risk of death. These changes can be attributed to development, genetic defects, the environment, disease, and the inborn aging process. The chance of death at a given age serves as a measure of the number of accumulated aging changes, that is, of physiologic age, and the rate of change of this measure, as the rate of aging. As living conditions in a population approach optimum, the curve of the logarithm of the chance of death versus age shifts towards a limit determined by the sum of (1) the irreducible contributions to the chance of death by aging changes that can be prevented to varying degrees, and (2) those due to the intrinsic aging process. In the developed countries living conditions are now near optimum, and the ALE-Bs are about 6-9 years less than the potential maximum of around 85 years. The inborn aging process is now the major risk factor for disease and death after about age 28. By age 28 only 1 to 2% of a cohort is dead, the remaining 98 to 99% die at an exponentially increasing rate determined by the aging process. This process ensures that few reach 100 years and none exceed about 122 years. Many theories have been advanced to account for the aging process. No single theory is generally accepted. Theories that can contribute to the important practical goal of increasing the healthy, useful span of humans will endure.