This is a family decision. There is no right answer that fits all. My Baseball son had a Sept birthday. We sent him to school with his class. We had no idea of this thing called baseball recruiting, or playing for varsity in HS. We had an idea that he would be relatively gifted, at whatever he decided to do, but heck he had only played Tee-Ball. I never played High School Sports, so this never came into our thought process, until he got into middle school, and we found out that most the kids in the class behind him were quite a bit older.
However by this time he was doing well in school both socially and academically. He was always a starter in Middle school. In fact there were many complaints from other parents on why he received so much playing time. Freshman year he was so much smaller than all the other players in his class. He still hit high in the line up, and started every game at first. (god only knows why, everybody else considered him an outfielder except the Freshman coach.) He was the first in his class to make varsity. Grew a lot from his freshman to senior year. Played and had a good experience at a D3 school.
Would he have played at D1 or a level higher than D3 had we held him back? I doubt it.
However that is just my Families story, it does not have to or should be yours. Judge what is best for your son and family. Had we known more about athletics we may have started him a year later. However I do not believe we could have gotten by-in from my child or the school system, had we tried it after he started school. School has very strict rules on holding back a student. And at the time I could not afford to send him to a private.