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.
unfortunately not every team or club plays by the rules 100% of the time. We played in a tournament last year and there was a club from Florida that had 2 or 3 older kids on each team. Our U15 coach thought something was up, but when they were at Subway having lunch they saw 2 of the kids they just played get into their own car and drive off. He made an official complaint and the tournament director investigated and found that this club had older kids on everyone of their teams. The entire club was thrown out of the tournament. Unfortunately for my sons U13 team we did not find out until we had already left the tournament. The director wanted us to come back, but we were to far away. I will never understand why anyone would want to win this way. If you need a trophy that bad you can go online and buy one.
