The Super Carnosine Pages. It is found in long-lived cells such as nerve and muscle cells. In fact, muscle levels of Carnosine correlate with maximum life span of animal species. Carnosine might be the most important anti-aging substance yet discovered. Most such claims are bogus but Carnosine appears to be a genuine rejuvenator.
Cells divide and keep dividing until they reach a limit and die.
When enough of the body's cells reach that limit, rapid degeneration and ultimately death ensue. As the cells age they undergo changes. Toward the last they become senescent. They assume odd shapes, do not place themselves properly in tissue, and they become granular in appearance. That's at the cellular level but aging affects the molecules that make up the body, too.