US Soccer mandates use of birth year for determining team formation and grouping. Rec soccer uses two year even number groupings based on players being under a designated age for that calendar year.
U = Under. Example: U12 = Under 12 years of age for the whole calendar year. A child that turns 12 ANYTIME during the year (yes, even December 31st after the season is over) would play in the next age bracket up since they are no longer UNDER 12 that year. As rec soccer follows two year brackets, in this example, the player would play in the U14 division, since they are under 14 years of age. Unfortunately there are absolutely no exemptions to this US Soccer law.