Let's all agree that parents who hold their child back for anything other than academics are scumbags but, in fairness, they are probably more shrewd than any of us. This is not about HS or college because, by then, multiple age groups are competing with one another. If this is purely based on youth sports then they figured out the secret sauce. Dominating the younger players on the field is just a means to an end. The goal is to get recognized and put into the "elite" athlete (even in youth sports) category because more opportunities present itself. Read Malcom Gladwell's book "Outliers" about youth hockey where kids born in January and February are disproportionately represented in the NHL and equivalent leagues. The reason is because these kids are bigger and stronger so get noticed early. These kids get chosen for higher level hockey and are on a different track then the younger kids within the same age group.