Age groups are now determined by the US Soccer Federation:

The date of birth must be as follows to be age eligible

Jan 1, 2012 or Later U-8   Jan 1, 2006 or Later U-14
Jan 1, 2010 or Later U-10   Jan 1, 2004 or Later U-16
Jan 1, 2008 or Later U-12   Jan 1, 2001 or Later U-19

BEST EXPLANATION: IF A PLAYER IS 12 PRIOR TO JANUARY 1 2020, THEY ARE INELIGIBLE FOR U-12 AND MUST PLAY U-14.  EVERY AGE GROUP FOLLOWS THE SAME CRITERIA.