Theories of Aging
All organisms will age and die. Human beings, as a sentient species, are perhaps the only creatures aware of their own mortality. Senescence, or biological aging, is a fact of existence, but that that has several competing theories of aging that seek to explain exactly how and why we grow old and die.
The two main theories of aging that explain senescence are either programmed or stochastic theories. Programmed theories are based upon the ideal that a sort of "biological clock" that affects the various body systems somehow regulates the aging process. The Reproductive-Cell Cycle Theory, for example, states that aging is caused by subtle changes in the hormonal signaling from the body's cells. Hormone levels change in humans, dysregulating cellular growth and development, leading to the aging process and eventual death.
Stochastic theories seek environmental causes. These environmental impacts cause accumulated damage to body systems, including cells, over the lifespan, including damage to the body's DNA structure, damage to organs and tissues as the result of free radicals in the body.
Most theories of aging see the entire process as a cascading failure of homeostasis, the process of genes that maintain and repair the body. Most of these theories have been applied to humans, but there is also significant study undertaken to attempt to explain the aging process in all animals.