While we did not intentionally do early entrance to K for middle kid, she was young for grade. She started in a private language immersion school in Pre-K. The birthday cutoff in the private school is December 31, but the local public school is September 30.
We always thought she was immature for her age, and she was really small. We asked the private school if she should repeat Pre-K (asked again in K), but they saw no need. I should add that this is the kid I declared "not-college-material" when she was four, so perhaps at the time it seemed that holding her back would help with academics.
When we moved her to the public school, we had her repeat 2nd grade. Academically, probably not a good thing because the private school was ahead of the public in most areas (writing, math, etc.). Socially was a different story. I think she was much better off in the "correct" grade for her birth date (in October). Still kind of immature, still small, but a better social fit. While many here may discount sports, I doubt she would have continued her main sport - and no way she would have picked up her other sport - if she was young for grade. She loves her sports and is in her fifth year of travel for her main sport.
I don't think middle kid is damaged for life. She is currently in 11th grade and seems to have made it this far without major "damage" (though I'm sure she'd claim that we parents damaged her in some way - but most 17 year olds will tell you that!).
The decision has a lot to do with the kid. My eldest was one of the youngest in her class (late August birthday). She was always fine with older kids, younger ones, same age...did not matter. Good luck and consider what is best for your son.
