different for every kid, my sons best friend repeated 8th grade at a private hs before coming back to our public hs.
He was young for his grade, played little league with kids a grade younger and all his friends were in the grade behind him. he was a late bloomer physically as well. They held him back for all of these reasons, and yes, athletics was part of the overall equation. I don't blame them in the least- it was right for their boy. He's a freshman at a d2 school and got his first start yesterday, he's well adjusted and has good grades- so far so good. Had they not held him back he would not be where he is now. worked out well for them.