I would hope at a certain point they reach the maturity level where they can have fun without needing a costume, cheap hair do (yes it is very simple what they do which is fine for the age group), and tons of glitter?? Are you upset that adults can't wear costumes in Disney?? Sorry, just don't get that. I'd rather have kids be safe. 99 out of 100 times it'd be an adult just having fun. 1 out of 100 times it'd be some creep just wanting to touch little kids. I'd rather not have the policy and keep that 1% from happening... I've worked with kids who have had crap like that happen to them. And it's horrible. So no, adults don't need costumes.
I'd also hope mom (or dad) would have the maturity, yes maturity, to realize they don't have to feel guilty about not getting to play dress up with their kid? I've never ever ever met an adult who has felt that because they couldn't be Minnie... Other than Halloween, I don't need a costume to have fun. Sorry, Id find the adult a little weird if they suddenly start feeling out of place without a costume considering the fad of even kids wearing costumes isnt that old and that the super majority do not ever wear a costume, including kids. That's way way over sensitive. And short sighted, considering again, plenty of other ways to be a kid. Sorry, but I'm finding your phrasing of "not allowed to be a kid" because of this one hypothetical instance quite silly. If they wanted, I guess they could throw a temper tantrum at not getting what they want. That would certainly be kid like. And Casey's is always waiting. Maybe said adult could even hope to get a yeast infection the way a 2 year old would. I'm sorry. I'm teasing. But again, you are telling me one, I mean one age restriction suddenly means telling the adult they can't be kid-like??
At a certain point that grown up kid should realize its ok, you can have fun and be a kid other ways without comprising children's safety as a whole. And let daughter have their own special moment the way they had it as a kid at BBB. It's not a big deal.
I get that Disney has been changing. Check my other posts. I'm certainly not afraid to be critical. But I'm really failing to buy your argument in the case of one restriction on age... Like I said, if it happens more and more, we can revisit this, but until then, you can be that inner child just fine elsewhere than BBB.