You could, you could also go to their house and go through the process for them, but in either case I don't think it's going to scale very well (rate-limiting would seem to be called for, maybe with 2FA as well, to mitigate this sort of thing and remove the possibilities for automation).

But sure, you could subvert it on a small scale, just as you can borrow someone else's driving license to register in 'normal' systems already. You could also register an account, validate it and then sell the login details, regardless of what proof of age scheme you use.

The point is the scheme is no worse at validation than asking for ID and it protects user privacy by keeping all ID details away from individual websites, which is the more important part IMHO.