"All would live long, but none would be old." So wrote Benjamin Franklin over 200 years ago.
It was he also, who wrote: "In this world nothing is certain but death and taxes."

Now, we all know why there are taxes. Sort of. As for death, and its precursor, aging, we - as a species - have never really looked into it. For thousands of years human beings have accepted as inevitable the decline of our latter years, and our ultimate demise. It's the way things were.
Our fate.

And still, the question is there: Why do we age? It is a question fundamental to our existence, and yet it is only recently that we have come to ask it at all. The question is fundamental because aging brings us all to a painful and unwelcome decay. And then our extinction - the end of our earthly existence. And yet for centuries we gave the process of aging little thought at all. To be sure, religions through the ages have considered aging and death. But in general, these considerations were from a metaphysical rather than a physical point of view.

Concerning the physical point of view, aging was regarded as inevitable, as inexorable as death itself.
It was the natural culmination of life and we could do nothing to stop it.

With the burgeoning of the biological sciences, particularly in the last forty or fifty years, the process of aging has finally come under close scrutiny. With this scrutiny have come numerous theories of aging, some broad in scope, some less so; some relevant to current research, some less so: