The government issuing everyone a smart ID that lets them attest anonymously to being of legal age would be better.

But there are age gate laws today, and calls to pass more of them. A hypothetical better way in the future shouldn’t excuse legally mandating a poor implementation today.

We could distribute scratch off cards to stores within a few months. It's incredibly low tech. I can't speak to elsewhere in the world but most (possibly all?) US states run lotteries.

If a given government body can't manage to stand up a web API to validate one time use codes within a few months then they clearly don't have the technical knowhow to manage smart IDs in a secure manner.

My point being that this either doesn't qualify as hypothetical, or if it does, then it indicates gross incompetence to an extent that precludes more complex solutions as a matter of course.