So then this is an easy problem. Issue liquor stores a terminal. Liquor guy checks licenses. If you're an adult, the clerk presses a button. A public key is generated and uploaded to a public list. You get a private key that shows you're an adult and is not tied to you. Regular laws that apply to liquor also apply to this private key QR code... You cannot give it to a minor or sell it without a license.

To view adult content, use the code to sign a thing. Content company sees the signed code, verifies against the public list and sends the content.

Privacy preserved, no adult content to kids... Easy.