All adults proof their identify multiple times per month: Every time they access digital health records, or when they use any electronic payment.

Just make Google/Apple reveal part of that data (age > x years) to websites and apps.

Boom, done. Privacy guarded. Easy.

>Every time they access digital health records, or when they use any electronic payment.

This is the internet.

Among those who were very familiar with it, the smartest money never started doing things like that.