These numbers represent the maximum playing age allowed on a particular team.
These rules contend to both U10 and U12 teams.
The "u" means "under."
Therefore, as an example a 12u team will have players no older than 12 years old, except a player that turns 13 during the Spring Season. A 12u team *can* have players younger than 12, but never older than 12.
Playing age is determined by the age of your child on May 01.