I mistrust maturity issues. Sometimes people just don't fit in with age mates. This may last their entire childhood or entire life and should not be used to deny them academic challenge.
This. DS was in a traditional age-based classroom at a daycare and even at 3, he felt like he did not fit in at all. We were starting to have some issues develop, especially at home. And all the kids he hung out were 1-2 years olders - in fact, it got a little awkward when he would gravitate to the older siblings of his same-age classmates and have nothing to do with the same age classmates.
We had him start at a gifted private school where his class was for 4-5 year olds (basically kindergarten plus) - and there, he fits in with peers that are within a year of him. He still likes hanging out with older kids too, but he loves his class because his peers are like him.
