Possibly, but I would first define what he is no longer being challenged by.
If he wins every aerial duel because he is physically ahead, adult football may give him opponents who force better timing, positioning and decision-making. That can be valuable.
But you can also challenge him at U18 by asking for more responsibility in possession, defending larger spaces, playing with a higher line, or dealing with quicker combinations rather than only direct duels.
If senior minutes are available, I would look for the right exposure rather than simply the highest level. Twenty meaningful minutes against an experienced striker can teach a lot.
Manage the total load and keep communication between coaches clear.
The question is not “is he too good for U18?”
It is “what problem will stretch him next?”
If the youth game no longer provides that problem consistently, controlled senior exposure becomes a sensible part of the development plan.
