Not a big fan of weight restrictions of any sort for FB.
Certainly, there are exceptions, but the norm for the most part is "big kids" are not agile and/or coordinated enough at 12 yrs old (or younger) to be any more dominant or able to inflict injury than anyone else on the field.
Regarding "learning the game/experience" perspective, round these parts:
Middle schools only play 6 games. Youth play 10. So from this perspective, youth is a better avenue for kids.
Plus (again, round these parts) for the most part, youth coaches are better coaches than MS coaches.