I think it best for players to develop with their own age group. A 9 or 10 year old is at a disadavantage when facing an 11 or 12 year pitcher. That being said there are some 9's or 10's that can play with the big boys but when it comes time to select all stars for tournament play they are not among the best, but the parents or league want them considered for the lower league all stars. The lower league parents will be upset when their son does not make the team and a 10 year old that played in the majors did.

I think it best for 11-12 and 9-10 leagues, but if there is a very good 10 year old who wants to play up, he should compete against his peers when all stars are selected.