It would be great for a US Lacrosse representative to confirm this, but as I understand it..........

Based on this link:
http://www.laxmagazine.com/high_sch...ens_game_committee_approves_youth_rules.

US Lacrosse will implement a new cut off date of August 31. This is a change from December 31.

This would mean that you must be under the age of the group as of August 31 prior to the next season. This is based on how they calculated previously as shown in this link:
http://www.uslacrosse.org/TopNav2Right/Rules/MensRules/BoysRules.aspx

For U-11 2012, the player must be 10 or under as of 8/31/2011.

For U-13 2012, the player must be 12 or under as of 8/31/2011

For U-15 2012, the player must be 14 or under as of 8/31/2011

There are many tournaments that do not follow these guidelines nor do they have to. However, to be sanctioned by US Lacrosse these guidelines must be used.