With my older 2020 I had read all of the advice on transition etc so we started using -8 in the cage at 12u, -5/wood in the cage at 13u. Got to 14u and we still only saw tournaments (mostly USSSA) where he didn't have to use BBCOR. Freshman year, in HS / 15u COMING UP is going to be the first time he actually required to use it, but for a few odd tournaments, and at 6'1 180 he's actually asking me for heavier more end loaded BBCOR than the first balanced one I got him. At 12u I would not have predicated this because his size was pretty average.
I coached through these age groups and saw MANY young undersized and large kids alike struggling with -5 Easton Xl1s (more like a -3!) BBCOR, wood when they didn't need to use it. Hurt their confidence and hurt the team big time because averages and power went way down. Also these folks wasted a LOT of money on expensive maple bats and having their -5s for games.
I will not tell others what to do with bats (very sensitive area!) but with my 2023 he will use heavier bats in BP starting at 12u and whatever bat he needs to get around on pitchers throwing gas until he is actually required to use a BBCOR or wood. If he doesn't grow as much as quickly as brother then he'll need to do more pushups when he's 14. But I doubt it will be a problem for all but the really undersized kid.