This could be run like rec football. Your age on August 1st determines the age group you play with. Show a state issued ID, end of story. They have teams at each age group, just like lacrosse. Until you reach 9th grade then do whatever you want because they have to play against older players no matter what.

It makes too much sense and the "elite" teams can't game the system to make themselves look good so it won't happen.