> Let's say that there's a Government Identity office that, when you turn 18, grants you an Adult Citizen ID# security key that lets you verify your identity digitally.

There's really no reason a non-SSN identification number couldn't be used for this. If SSNs weren't sensitive, you could just have a government endpoint that takes an SSN tells you whether the holder is over 18 (or "no, or the SSN is not valid).

The only challenge is knowing whether the number belongs the the person sitting in front of the computer. There's no way to stop a kid from using their parent's number. Which is sort of the whole problem with SSNs in the first place: their value largely comes from the fact that they're secret-ish. In effect, they're passwords.