Especially in the US where you can join the military and fight and die for your homeland at 18 but can't drink until you're 21. The same holds here, Yang is 17 and can train to fight Grimm, a large part of said training involving fighting said Grimm and yet, assuming drinking laws are the same in Vale as in the US she will get thrown out of a bar. It shouldn't be hard to understand why this is fucking stupid.
You seem to be focusing on the idea that 'can drink' and 'can fight and die for your country' are both connected to
adulthood to conclude that allowing the latter earlier than the former is 'fucking stupid'.
I'm not really sure why you'd assume adult-ness is the driving factor here. My personal experience suggests that, if any kind of underlying logic drives it, it has to do with the fact that teenagers tend to test boundaries and alcohol lowers inhibitions, and those two together leads to fairly out-there behavior in terms of seriously violating social norms. In RWBY's case, if we take the 'Grimm are drawn to negative emotions' idea seriously, I can totally buy that a culture living in such a world doesn't really want its already-volatile teenagers made even
more volatile, because of the dangers inherent therein.
For that matter, in a world with superpowers where
some citizens have their Aura activated and many don't seem to, I wouldn't be surprised if people card youth in general because there's no other sane way to keep alcohol out of the hands of your teenager superpowered people (You can't pass out a 'I'm not a Hunter' card to
everyone, that's insane, and a Hunter could just go somewhere nobody is going to recognize them), and a super-teen getting pissed could easily seriously hurt non-superpowered people without even
meaning to.
Mind, the real answer is probably that the writers just ported real-world laws and stuff in without thinking of any of it -if Vale were coherently handled as a realm on the brink of apocalypse thanks to the slavering hordes of demonic hellbeasts swarming just beyond the barely-holding walls, I'd honestly expect most any writer who thought about it for any time at all to come to the conclusion that Vale would much prefer its citizens drink heavily rather than think too hard about how shit the world they live in is, or something in that vein- but it's actually not any kind of
problem within the story, not by itself.
Yet.
(Note that I have zero opinions about drinking age laws myself. I don't drink because the stuff literally makes me ill, and I don't really care what other people get up to with alcohol so long as it doesn't bring trouble to me. Personally I'd prefer the thread stop derailing into sniping at American drinking laws and blah blah blah, and we go back to talking about RWBY, but if someone is going to
insist on trying to characterize me as an American troglodyte who is only okay with Vale's drinking laws because I'm such a savage, I'm preemptively shutting any such nonsense down:
I don't care about real-world drinking laws. We are talking about RWBY)