Hockey does it right........travel hockey is grouped by birth year; you are born in 2008 you play with and against 08's whether you are in 7th grade or 5th.
The problem w/lacrosse being by grade is the 'cutoff' is different in every state and then the so called gray months become different as well. MD is Sept 1, so anyone born in the summer can choose to be the oldest in the class or go early and be the youngest. Long Island is Dec 1st, so their gray months become Sept, Oct and Nov. Irregardless when we give parents the option and age doesn't matter but rather grade, we will always get 12 year olds playing against 14 year olds.
HS kids should be graduating at 17/18, not 19, but if they allow it then parents will do it for any edge. It's the nature of the beast and we can complain all we want, but it's never going to change. FYI college coaches get you for 4 years, they would rather have you at 20, 21, 22 and 23, than 18, 19, 20 and 21.