IMHO, the generalization that gifted kids are more mature isn't any more useful than the generalization that boys are less mature. What matters is your kid's maturity level, not anyone else's.

Last year, my DD was in class with a boy I'd assumed was gifted and very young for grade (he presented as extremely immature). It turned out that he was gifted and very old for grade, having been held back for maturity issues. Some kids are just really asynchronous.