So we do cater to the exception and place really young kids on senior teams, at the risk of possible burnout, more injury on less matured bodies, less appropriate social situations, and overall appeal of a routine?
While I agree that there are certainly some 12 year old anomalies, does it make sense to build our age grid around them? To drive, you have to be 16. To vote, you have to be 18. There are a few people who might be good drivers at 14 and some who are educated and reasonable enough to vote at that age, as well. But do we lower the age so those few can do what they're prepared for? No, because it's in far more people's best interest to keep it higher. On a far smaller scale, I believe the same concept should apply here.
This is not rhetorical, but do coaches most often choose their teams based on the athletes skill, or equal parts skill and readiness for a certain age division? I'd imagine it's mostly skill-based (could be wrong) and not always done with the athletes best physical and mental interest in mind (around here, we have a kinda biased crowd, because I'd venture to say that if a coach posts here, they probably have more knowledge and insight than the general "coaching public"). But in general, how are athletes chosen for a team?