yeah it's an interesting one
is it for development or to find out which franchise is the best up to a certain age?
if for development, is it best to have the kids play alongside and against a handful of seniors, or is it best to keep them playing with other kids?
most pro leagues run an under 21 squad as a reserve team that allows 3 to 5 over age players to play. it seems to be the norm for a mix of practical and developmental reasons