If we were talking strictly about recreational sports, it seems to me it would be reasonable to have co-ed teams based on size rather than on age/grade. My brother and I played on the same soccer team in a league at a Navy base one year, and he was two years older than I. (11 and 13, maybe?) Soccer isn’t the same as football, of course, but everyone’s the same size as *someone*, if you get what I mean.
However, school sports, in many areas, are big business. It’s all about winning, not about the good of the participants, no matter what sex they are.
Good point. As long as the sport is for the enjoyment and best overall development for the participants, and not some sort of insane adult-driven gladiatorial competition.