38 is the age they start to slip badly; Ali, Holmes, Tyson, Lewis, Holyfield. It just seems to be a cursed time to be a top flight Heavyweight.
Wlad will be on top as long as he's hungry enough and enjoys what he's doing. For the next 2-3 years age won't play a role. There's a higher risk he loses motivation than his biological age letting him down IMO.
Hope he doesn't get to the level where he looks diminished in skill and gets beaten by someone he would have destroyed. He never takes physical punishment thoguh so he should be good for a little while.