My kids have summer birthdays and were not held back. They are not atheletic but plays instruments. They were on the tall side but usually the youngest in the class. DS2 says he is THE youngest boy in his grade and some friends are 2+ years older than he is. He was the only freshman in his math class with sophomores and did fine. He will continue to be THE youngest in his math classes by 3+ years. Both kids will graduate HS at 17.