17 and 8M. I think if the kid is on the young side Aug-Sep birth date then holding him back say between kindergarten and first is a legitimate thing to do. But a kid whose average age or older and then doing it for an advantage, kind of silly, unless there are maturity, academic, medical or mental illness issues to consider. I think it gets a bit much when it's done between middle school and high school and then again by reclassifying later, for no other reason than a sport, and graduating at 20 years old. (you better be going pro and not just getting a 25% scholarship to make up the private school cost)
We considered all these options based on his age, and wish we had done it early since he didn't have public kindergarten the year he entered school age but it was offered the very next year. Would I have felt guilty about it sports wise, no because he would have been no older on average in his new class than some in his "legitimate" current classmates are over him.