I suspect my son would have been in that situation if we had stuck with traditional schooling. He was the youngest in the grade (we put him in private to get around the age cutoff), and ya know what -- no one suggested he was too young for it.
Immaturity issues can actually stem from him being bored, being forced to "socialize" (ha!) with kids who really aren't thinking about many of the things he is.
Even the private school my son attended most of his schooled years (we started homeschooling after 5th grade) wasn't the ideal setting for him. He learned to not focus (getting the work done wasn't a rewarding experience, and his thoughts were more interesting than the work - I truly wish I had homeschooled sooner!). We have, after 4 years, finally gotten past a lot of the issues we had that I think developed out of being under-engaged in school.
I agree w/ Trinity, that getting him into a different social environment is important.
