different disctricts within states as well for enrolling in pre k or kindergarten. and it rolls up to international play, different countries have wildly different cutoff dates for starting school (in US in my experience it is somewhere from Aug - Oct), so the international cutoffs are all generally Jan 1, so youth sports match.I have always wondered why softball classification were not based on school years so girls in the same grade could play with one another. I guess it is probably because the states have different cut off dates for enrollment.
older DD (does not play softball since 12u rec) has experienced this, one of the oldest in her class always, generally always one of the youngest on her non school sports teams. has really only helped her, and she normally played up anyway (been playing 19u FH pretty much since 8th grade).