I get that you want kids on level appropriate teams, but I mean, that will kill the small gyms. Say a gym is in its first/second year and has 30 athletes. They probably aren't all the same age. I know for a fact that this would hurt my gym... We have some junior-aged girls on our senior team because, well, we only have 2 teams, and we're trying to let them throw their level three tumbling (even though a couple have level 4/5 tumbling and stunting, this is the best we can do). Should they have to compete on a level 2? No, that's not fair to them, and it's not really fair to other teams to have them competing down.
Plus, there's the whole "sandbagging" thing. By putting bottom-ages on divisions, small gyms are going to be accused of "sandbagging." If they have a level 4 tumbler who's 1o years old, and their only youth team is a level 2, it looks like they're sandbagging. Small gyms already get shamed for sandbagging, because it's already hard to have 2 teams and have everyone on a level appropriate team, and it'll only be worse if ages are stricter.