The Waste Accumulation Theory posits that as we age our cells accumulate more waste products like lipofuscins and spherons than they can ultimately dispose of.

In our younger years our physiology is such that anabolism (constructive metabolism) outstrips catabolism (destructive metabolism). Thus, at the height of our youth we have bone, muscle and other tissue being built up more quickly than it is being broken down. Waste products that result from these processes are easily disposed of by our vigorous, youthful cells and detoxification systems.
Later in life, the situation is reversed.

The result - there is an eventual accumulation of cellular waste products: Biosludge.

There is evidence to support this theory.

For example, the cells of the body contain a waste product called lipofuscin, or age pigment. This age pigment occurs in cells as small granules, but the granules grow larger as a person ages. In the skin it is this substance that gives rise to the age spots that start to appear around age 40, and thereafter accompany our advancing years.

More damning, lipofuscin is found most commonly in the cells of nerves and in heart muscle, such that brain and heart function deteriorate with time.