Was your child one of the older kids or one of the younger kids in their class? How did that affect them? How did it affect your relationship with them? My S. was older, since we were advised to hold him (wayyyyyy back in prekindergarten). H. and I had both been very young and saw the wisdom in letting him be older, although we were very torn about what to do. Finally we realized we had each always felt socially out of it and once this decision came into play, hypothesized that age relative to peers might have contributed to that.
It has been an interesting road. He's often been friends with the kids a grade ahead, as well as in his class. He's not at the tippy top of his class, but has done a very good job overall. Twelve years later, here we are at graduation. Last year I wanted him out of the house yesterday (16 and 17 were tough ages), and now at 18 he's calmer, happier and generally more civilized. More mature, I'd guess you'd say =). So in the end, I'm glad we held him - glad to have been able to send the maturer kid out into the world.