It's insanely subjective. I think honestly the biggest crime here is when people who are too old play teenagers. It's so cringey, and then I think it also gives teenagers unrealistic expectations of what their bodies and voices should sound like. I understand that the reasons this is in place is because of the liability/headache of working with actors under 18, which I understand all too well as a director myself but I wish we could maybe adjust some of the laws to make it easier. I have sometimes put strict height restrictions on calls requiring these roles so that it looks more believable--even though I know there are several teens out there that are about 6 foot. But, I don't think it does anyone any favors to see actors in their 30's play teenagers on stage. Generally, Spring Awakening was pretty good about this--but like, recently I saw The Mad Ones and all of those actors were basically in their 30's. It was so stupid, and besides K&L being total hacks and that show being one of the pinnacle reasons why I think modern musical theatre is so insipid; it would've at least felt more authentic to have had younger actors who passed a lot better.
I do think like I said earlier though--everything its in context. Like, there are some theatre legends out there that can play a role up until they are wayyy past their prime, and we, as audience members forgive the rules of the world and allow it because our want to be entertained is much greater than our need to be let into the dramaturgy of the story. Like, to see Ted Neeley in JCS, Ellen Greene in Little Shop, Richard O'Brian in Rocky Horror, Bette as Dolly--these people are so iconic that the only people who would be taken out of it is those unfamiliar with those people as performers, or the show being performed, and most of the time it's diehard MT fans who could give a crap less. But again, there are some limitations--such as when a role requires revealing clothing or nudity that it might not be in the best interest of the performer. But the again, Ellen Greene for instance when they did the Little Shop concert looked incredible. So who knows?