Funny thing is that the legal drinking limit in a lot of countries is 18. I think it's even lower in some, like 16 in Germany or something like that. I don't understand why it's 21 in the US.
It's 21 because when it was 18, a lot of teenagers would get their license and drink & drive so there was a lot of car accidents & deaths so they figured that when you're 18 you're a bit more mature & won't recklessly drive drunk
The picture is just a reaction