Hopefully these age guidelines will become the norm but club teams, tournaments have a lot invested in grade system. USL makes guidelines but doesn’t have power to force on tournaments or leagues they don’t directly run. They’ve always advocated age guidelines but with 2 year gaps u11,u13 etc. So this is only a start and it will be interesting to see if clubs and tournaments follow.

also i think this is for youth lacrosse not HS. So in the case of a fall bday in NY, he'd be among the older kids (at least locally) and play with and against kids in the grade below during his youth days. Then in 9th grade become one of the youngest. Could be good or bad for that kid depending on your viewpoint