Depends on the style of heavyweight. Big heavyweights, who don't rely on speed do well to an older age. Foreman, the Klitchko's, Lennox for example.
Heavyweights who rely on speed start dropping off in their early to mid-30's. Ali, Tyson (other factors too of course, but he was noticeably slower), Roy Jones Jr (not a heavyweight but a glaring example of a speed based fighter who started losing horribly after age 34) and yes, Fedor.
And even when he was 32, before he started losing, his boxing coach said he was slowing down noticeably (but he would make up for it in other ways). When a coach admits that it means he knows its so noticeable that there's no point in denying it - and its pretty clear watching him at that point. His hands still move fast, but the reflexes, what he used to not get hit (its how he, like Roy Jones Jr got away with the low hands), had decreased noticeably ... as they say in sports, he'd lost that half-step on his opponent (meaning they get a half step before he could react).
Fedor's style is bad for an old man. Unless he's revamped it, he's in for a world of hurt, like 38 yr old Ali losing to Trevor Berbick, a fighter who wouldn't have lasted a couple of rounds with Ali in his prime.
So I guess that's the question. During his time off, did Fedor change his style from speed/reflex based to the kind of slow the pace down/grinding style that works much better for older fighters?