The year and month a child was born determines in what age bracket the child should play. US Youth Soccer & Alabama Youth Soccer Association run their age brackets from August 1st of one year to July 31th of the following year. Other sports have different "years." Schools also use different "years" for placing children in a grade. Very few clubs/associations have an Under-4 bracket. Under-20 and older is considered "Amateur" and not part of "youth" soccer.
Clubs/Associations that do not have enough players for single age bracket teams usually combine age brackets into U05/U06, U07/U08, U09/U10, U11/U12, U13/U14, U15/U16 & U17/U18/U19. A team has to play in the age bracket of its oldest player. So if a team consisted of 3 Under-5 players, 5 Under-6 players and 1 Under-7 player, the team would have to play in an Under-7 bracket. Players can play "up" but may not play "down."
A soccer year consist of two seasons, a fall season and a spring season. Some clubs/association have teams only in one season but most have teams in both seasons. The spring season team is usually the same as the fall season team, less those players who drop out of soccer in the spring, who are usually replaced by new player who didn't play in the fall.