You may have already heard this advice but take a look at the Iowa Acceleration Scale. It was very helpful to us because it gives you a number and puts that on a scale of "excellent candidate" to "don't do this". It does require an IQ test and above level testing.
We skipped our son from 1st to 3rd just this past school year at a small private school. He turned 7 toward the end of July and all the other kids in his class (there were only 4) turned 9 in September. So, I thought that might be an issue but it never seemed to be. I think he did miss having recess. Being in a small class helped ease the transition, I think, although the school was still unable to meet his academic needs.
We are moving to a school district with a better gifted program. We'll have to see how he does in a class of 22+ kids who are all older than he is. I think he'll be fine but I do worry some about it.
I started K when I was only 4 (instead of the required 5) and did have some social issues in junior high. HOWEVER, I was so shy that I'm pretty sure I would have had those same issues whether I was the same age as the other kids or not. I did get my driver's license later and went off to college at 17 and couldn't get a phone in my dorm room because I wasn't 18. No big deal. I think a large part of it does depend on the child's personality. It's not a one-size-fits-all decision.
