Just for a counterpoint to the "17 when starting college" thing, sometimes that works out fine. My sister was skipped a grade from K to 1st when she was 5, and she was 17 when she graduated. She went to a good college, had no trouble adjusting, and is very successful in both her social and professional life. She was also very athletic and had no trouble keeping up with her sports teams in school.
I was 16 when I went to college and lived in the dorms, and my birthday was in April so I didn't turn 17 until almost the end of my first year of college. Socially and academically I did fine, although some people did make comments about me for being so young. Being 16 didn't really affect me as far as being on my own, either. Dorms are like an extension of home -- there are other people around, a cafeteria, etc. I was 17 when I got my first apartment and that was fine too.

I really think it's important to take things on an individual basis if you can. But if you do end up having to start him late, and you think he's bored/unchallenged/much more mature than the other kids, you can always look into moving him forward or enrolling him in a different program. I know there are lots of pros and cons re: skipping grades, so again it's really going to depend on the kid in question whether it's the right thing to do or not. I just mention it because I remember when DD1 was just starting school -- I sweated bullets over every decision I made because I thought they would all have some huge effect on her future. But in reality, it's a continuum, and life and schooling are pretty flexible... especially in the early years. Now that she's in high school I realize I didn't have to worry as much as I did.