Second if fairness is the sole guiding principle behind youth athletics, we really would differentiate based on not only capabilities but also by height, weight and maturity (which are the prime differentiators at the youth level). There is nothing fair about having the early developed, 6 foot 12 year old competing against my son's friend whose back was broken when he was little and now is uncommonly short but is pound for pound a better soccer player than the 6 foot 12 year old that just muscles his way through the game. We do, after all, differentiate in boxing, wrestling and martial arts by weight classifications. There are numerous problems with this, however: 1. fairness is not the only guidepost but also participation (hence the women's division, and hence why some people argue trans people need to be accommodated somehow in order to be able to participate, rather than just thrown in the men's division where they can't), 2. the other guiding principle we have is ease of administration (can you imagine the nightmare of sorting kids that are growing into respective groups...teams would never hold together and each tryout season would be a mess) and simplicity, 3. there would be mass objections when their little Johnny or Mary precocious 12 year old winds up being assigned to play with the 14 year olds and goes from superstar to merely above average, and 4. there would still be some unfairness in the process, as like in boxing, the main event people care about is the highest levels: the heavyweight champions of the world, though not necessarily exclusively (sports is, in some ways about being the best of the best and the women's division only makes sense from a participatory rationale otherwise we'd be living in Starship Troopers)