"probably unconstitutional under the First Amendment, to boot"

Probably not. Minors have all sorts of restrictions on rights, including first amendment restrictions such as in schools.

"(And if it is, let's start talking about gun ownership first...)"

Are you advocating for removing ID checks for this? If not, it seems that this point actually works against your argument.

Not saying that I agree with a ban, but your arguments against it don't really stand.