It all depends on the kid and circumstances. It could help development if they're advanced for their age, or it could be a hindrance with them spending too much time on the bench. DD played up through 10U because she was capable, especially on defense, but also because it was more convenient with her older sister on the same team that I was coaching. However, being on the younger side for her birth year, I ended up holding her at 10U for an extra season to allow her to catch up in size and experiment with pitching lessons. After that, she played within her age group.
I'd be careful about putting a 12U on a 14U or older team. The differences can be significant both on the field and in the dugout. Putting a 16U on an 18U team isn't a big deal, but if you're looking for something beyond JUCO, being on a good 16U team that plays in the right tournaments is the way to go.
I'd be careful about putting a 12U on a 14U or older team. The differences can be significant both on the field and in the dugout. Putting a 16U on an 18U team isn't a big deal, but if you're looking for something beyond JUCO, being on a good 16U team that plays in the right tournaments is the way to go.