What kids are being hurt the kids on age or the kids being told by their parents they are not good enough to compete among kids their own age. The difference between a 14 yo and 15 yo physically and maturation wise is dramatic but not as dramatic with a 18 and 19 yo. The kids who were never moved up are less prone to injury while the kids s held back are used to being the bigger and stronger kids will not be able to handle the strength and pressure of being the younger kids when in Hs and college. Look at college and I bet more star players were never held back. Just read outliers to see the holding back concept fails as the kids are all matured. "You can always play up but never play down". I have no problem letting my kid playing with older kids. It will only make him stronger