It might be Fort Knox just fine at some point, when computers will require a cryptographically signed government certificate that you're over 18, and you can't use the computer until you provide it.
Even in that case the large majority of the population would then have that certificate and the motivated minors would just beg, borrow or steal one.
No one has ever faked a government ID?
Even in that case the large majority of the population would then have that certificate and the motivated minors would just beg, borrow or steal one.