They've all been playing since they're 8. It's toughest to succeed in the NHL, but you're skating just as hard and taking the same kind of beating in the AHL, the Swedish elite league, the KHL, or wherever, at least if we're talking about mileage.
No you're not. You're playing against smaller, slower people and on a shorter schedule. I'd be curious to look at this - how what year a player starts his career might affect what year he ends it - but one of the problems with measuring this is survivorship bias - rarely do players as not-great as Zubrus start in the NHL at 18. And of course career-ending injuries are a thing.