If the player, the DOC, the coach, and the parents are OK with the player playing up, then you have a good consensus for them to do it. Do not be offended though if and when the player is moved back down to their age group at some point. Usually around U13/14 this will happen. You will also see some players coming up at this time. It's all about the timing of their physical development. I've seen parents switch to inferior clubs just so their kid can keep playing up. I think that is very short sighted in the long run.

I don't think I'd want my u12 on the u13 team full time, but he will be playing with them for a few games for the experience of 11v11 and for more game time.