Despite the best genetic makeup and medical care, no one seems to live much beyond 120 years. Every species ages, undergoing noticeable changes from birth to death. Scientists have developed theories for why people age, although no theory has been proven. Ultimately, parts of each theory may explain why people grow old and die. With the programmed senescence theory, the rate at which a species grows old is predetermined by its genes.
Genes determine how long cells live.
As cells die, organs begin to malfunction and eventually cannot maintain the biologic functions necessary to sustain life. The free-radical theory says that cells age as a result of accumulated harm from ongoing chemical reactions within cells. During these ongoing chemical reactions, toxins called free radicals are produced. The free radicals ultimately damage the cells and cause a person to age. With age, more and more damage is done until many cells cannot function normally or have died.