Not even sure what to make of the phrase “double holdback”. My son was held back in kindergarten (i assure you it was not for lacrosse). If he were to become a “double holdback” and repeat another year, he’d be ineligible to play in the Washington or Baltimore private school leagues by his senior year (which are supposedly the king of the holdbacks). Not sure what the LI rules are. Not saying it’s not an advantage, by the way, but it is limited. Also when you hit puberty matters, so does natural size and speed, and AA teams tend to attract those types which maybe amplifies the difference for parents of smaller kids or late blooming kids.