Is training up an age group actually useful for development?

I wouldn't view a move up an age group as a trial or a reward; I would treat it strictly as a development intervention. The danger with 'playing up' is that we often mistake physical dominance for technical ceiling. Before moving him, I’d want to see him demonstrate consistent 'football IQ' in his current age group—specifically, evidence that he is scanning early, making independent decisions under moderate pressure, and solving problems before the ball arrives. If he is already doing those things effectively, the move to U12s is useful only if it forces him to process information faster, not just battle physically. To keep his home environment feeling meaningful, I would treat these sessions as 'guest appearances'—perhaps once every three weeks—rather than a regular rotation. The focus should be on how he adapts his movement patterns when he has less time on the ball, rather than how he measures up against older, bigger kids. If the experience makes him more clinical and decisive when he returns to his own team, it is working.