I'm reading up on longevity & paleontology (of mammals). My armchair theory of the day is: we get old and die because of our teeth. For literally billions of years, mammals create 2 sets of teeth for life (the have to fit together perfectly, so they are irreplaceable) Once your 2nd set are used up, you die. Everything else in your body evolved to work just well enough to last as long as your teeth.
Sharks (new teeth all the time) can live up to 400 years.