You cannot conflate the two, at least not in this setting. Parents are usually making the call to start their child in kindergarten at a certain age because of their level of readiness not that it might give them a possible advantage in a hypothetical sport they may play in the future.

And age does not automatically give an advantage I’ve seen plenty of younger age squads run older teams right off the field. And the ones that do get held back specifically for sports is typically because their game is missing something.