1) His strenghts were the kind that do not diminish with age- size and power. 2) Speed is the first thing to go, and as he said himself, he never was fast in the first place. 3) he learned to relax and pace himself 4) He never took a lot of punishment in his first career, so he was youthful.
Popularity, careful match making, determination, and in some cases favorable judging played a role too. The comeback Foreman learned how to relax in the ring more, and made better use of his jab. I do not think Foreman in the 1990's was better than Tyson, Holyfield, or Lewis. He was more on the level of Mercer, Morrison, and Holmes
Considering he lost 10 out of 12 rounds against Morrison, I would say he was not on the level of Tommy.
For Holmes, I would argue that ring rust was more of a factor than age. On the basis of Holmes-Mercer, I'd register: Holmes UD 12 Age On the basis of Holmes-Beanfart, I'd claim: Holmes UD 10 Age. The fact is that Larry did go out a winner, as Foreman deserved to with Briggs. Holyfield's not finished yet, and he's still winning.
Over the early stages of his comeback, George had Moore training him again, and Archie knew a thing or two about competing successfully at an advanced age. Foreman's jab was far more accurate than it was during his first career, and it wasn't unusual for him to land it 50% of the time or more. I think he was much stronger physically while in his 40s than he was in his 20s. He was seen to move in reverse a bit during his youth, but hardly ever took anything resembling a backwards step as an older man. (Somewhat ironic, since he frequently went to his toes and practiced movement in training.)