The player has to be 10 as of January 1, 2009 in order to play 10U. So, there will be 11 YOA kids on some 10U team. To play 12U, the player has to be 12 as of January 1, 2009. Thus, some kids will be 13 YOA on 12U teams.
This doesn't apply to you but I just wanted to chime in that things sometimes get confusing for me where I have one DD that is a New Years Baby.
Some jurisdictions go by her age on 1/1 others go by her age on 12/31.
Fortunately it hasn't been a problem so far.