Another is that our neurologic and hormonal systems wear out over time, making us more susceptible to a variety of diseases.
A third hypothesis is the "wear-and-tear" theory - that living itself makes us old.
A fourth theory is that over the years the body eventually accumulates so many toxins and waste products that its systems begin to shut down.
You might know the fifth theory as the "free radical theory of aging": The body builds up free radical "oxidants" that damage the organs and DNA (deoxyribonucleic acid), causing us to age.
A related theory is the glucose toxicity theory, which also involves waste buildup in the body.
The seventh theory of aging derives from the law of entropy: Within the universe there is continual movement from order to disorder, and, in our bodies, that movement is marked as aging.
The eighth theory, the mitochondrial aging theory, relates to the energy sources of our cells and to the second collection of DNA that you and I have in most cells. Most cells have DNA, both in their nucleus and in another structure in the cell. Simply stated, our cells are in a constant state of division and renewal, during which mistakes can occur. And mistakes in the cellular (non-nuclear) DNA, which are called mitochondria and which govern an energy transformation process, may have a big effect on our health and rate of aging. For example, our DNA is continually being replicated, and when we don't have enough folate -- vitamins B6 and B12 -- the replication can occur with errors and abnormalities (which can possibly lead to cancer). The great news is that some genes, drugs, or supplements may be able to keep the mitochondrial DNA much younger.
Find out more about this book:The RealAge Makeover: Take Years off Your Looks and Add Them to Your Life