I'll chime in...We held all our kids back one year when we moved back to the USA - they are all summer birthday boys. One is gifted (really, not just because I held him back), one is bright/normal but not gifted, and one is normal/slow
1. I didn't want them as the 15 year old passenger driving with a 16 year old classmate whom I do not trust. I'd much rather they be the 16 year old driver. If I do not trust my 16 year old driving, then I guess I have big problems.
2. I didn't want them to be the smallest child, physically, in the class. And, no, not because of sports. My kids have a 0 percent chance of being on any sports team (5% height and weight). They are bi-racial and we have a minority religion so I am worried about bullying.
3. I wanted them slightly older when they leave home for good (this one is totally selfish - I like them and am not in a hurry for them to leave - at least I admit it)
4. We live in Texas - it is the norm here (so yeah "everyone else is doing it..." it sounds lame - but it is a consideration to have your child not be considered normal - again, we are already not normal in so many ways)
5. The normal/slow one struggled in his prior school so I figured with the move it would be a good time for him to reset his feelings/insecurities around school.
6. My father graduated very early and had mentioned to me that while academically he did very well (debatable, IMHO - he can't write worth squat) he always struggled socially. I asked if he struggled only in elementary, but he mentioned that the difficulties continued into college. That said, some people just struggle - maybe he was one of those.
7. I wanted to give them a longer time to think of themselves as "American" before they go off on their own (one thinks he is Chinese, one thinks he is Arab - neither me nor my husband is either of those - kinda weird)
Results - no regrets with either the normal/slow or the normal/bright one. I really don't know with the gifted one yet.