While aging processes can be divided into three general categories -- genetic, biochemical, and physiological, theories of aging fall into two categories.

The "programmed" theories hold that aging follows a biological timetable, perhaps a continuation of the one that regulates childhood growth and development. The damage or error theories emphasize environmental assaults to our systems that gradually cause things to go wrong. It's important to note, many of the theories of aging are not mutually exclusive.

Here is a brief and very simplified rundown of the major theories.

Programmed Theories

• Programmed Senescence. Aging is the result of the sequential switching on and off of certain genes, with senescence being defined as the time when age-associated deficits are manifested.
• Endocrine Theory. Biological clocks act through hormones to control the pace of aging.
• Immunological Theory. A programmed decline in immune system functions leads to an increased vulnerability to infectious disease and thus aging and death.
  

Error Theories

• Wear and Tear. Cells and tissues have vital parts that wear out.
• Rate of Living. The greater an organism's rate of oxygen basal metabolism, the shorter its life span.
• Crosslinking. An accumulation of crosslinked proteins damages cells and tissues, slowing down bodily processes.
• Free Radicals. Accumulated damage caused by oxygen radicals causes cells and eventually organs to stop functioning.
• Error Catastrophe. Damage to mechanisms that synthesize proteins results in faulty proteins which accumulate to a level that causes catastrophic damage to cells, tissues, and organs.
• Somatic Mutation. Genetic mutations occur and accumulate with increasing age, causing cells to deteriorate and malfunction.