Comment by flumpcakes

This is the exact system I suggested to a friend. I don't mind having to 'prove' my age, but I do not really want a third party to have my identifiable information nor do I really want the Government knowing what fetishes I may or may not have.

For a digital only solution, I think the best system would be some form of public-private key attestation:

The government advertises their public keys for 18+ verification.

A website generates a unique token - this token is then taken by the user and submitted to the government receiving a signed attestation. This can then be given back to the website to prove the user is 18. It only has to be done once per profile and no information is shared between the Government and the website on who is who.

Unless of course the token is saved by both the website and government in some forever database and then a lookup is done.

Another solution could be a timed/signed token produced by the government that has no input from the website. But this still has the downside that this could just be saved by both parties and in future you could identified if both sides compare data.