She died at 121. And most of the theories aren't exclusive. Evolutionary aging Nature designed animals to reproduce. Get beyond the normal age of reproduction and you're less important in the game of natural selection, Hayflick said. So a basic underpinning of longevity theories is that after middle age, humans live on excess capacity.
Cheap watches provide a good analogy.
Now most of the watches, of course, will last a year plus a day or another month or even another year. But that's a function of design and manufacture of the watch. However, you don't build into the watch a mechanism that kills it on the 366th day because that's too expensive. The same thing analogously is true in humans and other animals because nature's very economical. She doesn't build into you and me a device that'll kill us when we're 35 or 40.