The tournaments normally govern the age, I can tell you my son who is a sept birthday and started school early plays on a 2017 team but is the same age as boys on the 2018 team you mention
When a tournament is age based, he plays on the team, when its grade based, they do not allow him to play so I know for a fact at least one of the teams you mentioned here is following the tournament/league rules for age and not stacking teams. | 
