My DS, who is currently in 9th grade, started kindergarten a year early and skipped 4th, putting him two years ahead. He's in a small school with multi-age classrooms, where this is not unusual, so it has not been a problem for him academically or socially. He's still at the top of his class and may skip another year in a subject that's in his area of strength. He's got close friends and tells me when the subject comes up that he cannot imagine being two years "behind" where he is. I've also had his teachers tell me that they never would have believed someone his age could have the social and emotional intelligence and maturity that he shows in his writing. Writing and languages have never been a problem for him.
I think it all depends on the kid and the setting. Knowing your kid and trusting them and your gut is everything.
