Minnesota’s ban on 18- to 20-year-olds obtaining a permit to publicly carry a handgun violates the US Constitution, a federal appeals court ruled Tuesday.
I think you’re reading a bit too much into that amendment. It doesn’t say anything about individual rights, or the government infringing on them. It simply repeals the 18th and all related laws.
No. Supposedly they raised the age to 21 because of drunk driving teens, though I'm not sure what effect it had. In my opinion, the better option regardless is to introduce teens to how to use alcohol responsibly and safely, rather than prohibit until it's too late.
It didn't have an effect, it was never the teenagers doing most of it. When they studied the fall off that happened it turned out that public awareness campaigns were far more effective than bans or punishment.