I would discount injuries which are a whole different factor. I think other players and coaches figure out what the young player can and cannot do. Where their strengths and weaknesses are.

In baseball an example of such a player is Jeff Francoeur of the Braves. He had a great first two seasons then his production dropped dramatically in his third year never to recover. They watch films on players and change how they approach them. It goes for all sports. They start to take away what makes a player successful and that player has to adjust. They force them to do something else, to change their timing and the defensive looks they get. Some players can’t adjust, their talent only goes so far.