There is a certain genetic component with that kind of health too, usually there will be SOMETHING that breaks at that age. I know I will have some issues, because things that hurt or were minor problems in my 20s and 30s are not getting any better. I have a curved spine, so my minor lower back problems will make it hurt to get around when I'm 80 at least a little bit. There are also all kinds of degenerative diseases that don't start showing up until later in life. Fats just never get to live long enough to experience them, just like people in middle ages never got to live long enough to get those either, but they died of injuries, wars, starvation or communicable diseases, not beetus.