Reply I got from US Lax
US Lacrosse promotes single age groupings as the best practice for youth lacrosse. Sometimes this is not possible due to a lack of players in a certain area and sometimes it is done for a competitive advantage. Children of the same age are generally of similar physical and mental ability. I would encourage you to contact your league or club to adopt the US Lacrosse recommendation of grouping by a single years, like U10, U11 and U12. As the lacrosse consumer you have the power to tell the market what you prefer as the person funding your son.
You may be interested to know that US Lacrosse will be making an effort for age verification and event sanctioning which will help with some of this issue. Please see the link below. | 
