NHL players tend to enter their prime by their mid-20s. While it's very possible to see some spectacular performances before that age—Alex Ovechkin scoring 106 points at 20, Sidney Crosby winning the Stanley Cup at 21 and so on—it's around 24 or 25 where the "kid" is fully gone and has been replaced by a grown man.
Now look at them: Kane's a two-time Stanley Cup Champion, while Weber sports some of the manliest beards ever come playoff time.
All of the aforementioned players continue to be outstanding players because of the work that they put in over the years. Sure, it helps that Kane grew into his body and has packed on 20 pounds since being drafted in 2007.
What really sets them apart, though, has been their willingness to work on their weaknesses. Crosby wasn't always good at faceoffs. Ovechkin turned into a one-trick pony on the left wing before thriving on the right side last year.
The top prospects in the game today aren't any different. Seth Jones, Nathan MacKinnon and so on all have things that they need to work on if they hope to become truly elite players at the NHL level.