As discussed ad nauseum - the final rule, whatever it turns out to be, doesn't have to be exclusively grade based, or exclusively age based. As only an example, basketball uses school year, but augments that with rules that the kid has to be born after a certain date, in addition to their grade. This means that no, you can't have a 15 year old 6th grader on a 6th grade team.
But it does certainly add some necessary complication, because now the player has to be in a certain grade, and also has to be a certain age (or younger). This is where it often falls apart in basketball, because not only do different tournaments/leagues have different cutoff dates, from Aug 1 to Sep 15, but they also have different rules for holdbacks. So in some cases an eighth grader can play on a seventh grade team, if they meet the cutoff for seventh grade age. But if they are actually in 7th grade, they don't have to meet the 7th grade age, and can be as old as 8th, or sometimes even 9th grade age. And complicating all of this, is that in almost all cases - nobody really follows these to the letter, and kids of ages/grades that they probably shouldn't be are on the court all of the time - it's just something that you either get used to and tolerate, or are continuously annoyed at the perceived cheating.
That is one reason that the BY is clearly superior, in that it's simple to both explain and enforce. Kid born in 2012? Great - they are on a 2012 team. If the new rule is still just as simple, and DOB-only, with a cutoff of Aug 1, then it's still not terribly hard to explain or enforce - thought switching costs are certainly very high.