Already at 42, Tom Brady is a wonder.
To continue to play at the high level he does is, to me, amazing. I don't know that he's still the best QB in the league, but if he's not I'm not sure who's better.
I think that the longer a QB can play, staying healthy, loving the game, taking good care of his body, the better he gets..... to a point. The knowledge of the game, seeing defenses, recognizing situations and coverages, etc., only comes with time and experience.
But, at some point the body, reaction time, etc. simply gets so far beyond prime that it can no longer compete at the highest level.
I have to wonder just how much longer that can be for Tom Brady? Can he stay on top of the game until he's 45? Or even more? It seems to me that of all positions, the QB has the opportunity to play the longest because his knowledge and technique can overcome a somewhat diminishing athletic ability.
I have no idea because he's already breaking new ground. And since he's not Superman, that means that others can do it as well.