Why would an 18yo need more maintenance if nothing affects development?
Not sure i understand your question?
I will still answer my best.
Many things affect a player development and its mainly up to the player in question.
A player don't become worst because he struggle in the NHL. A player don't become better because he dominate a lower tier league.
An 18YO require maintenance because everything is new for him. The pace, the season long grind, the practice, professional routine, plane travel, every town and amphiteater, etc. Whereas a veteran dont need coaching or care. He is experienced.
You have Pearson and he instantly arrive and fit wherever you place him, whatever the role you have for him. Nothing is new and he dont need babysitting.
Roy might not take well a demotion, may question himself over a simple line change, every town will be new, he may not grasp everything in practices, etc. And its normal, its part of the process and every player goes through it.
That is frustrating for us fans but its something important and its taken into consideration by the coaches.