There's lots of cryptographic type approaches where the entity validating you is air-gapped from the entity certifying your age.

But if you don't trust them it's not hard to figure out a scratchcard system where for, say, £1 cash your local newsagent checks your ID and lets you pick a card that you scratch off to get a code that you can then use to obtain a cryptographic token online signed by a recognized CA. Neither the newsagent nor the card issuer have any way of tying you to that code, and if you don't like the idea of using the same token on multiple sites you can always buy more. Of course you'd also have the option of obtaining codes online, but there's something I think people would find reassuring about the existence of a visible physical gap.

In my country if you want to buy booze online, you verify your age by logging into this id check service the banks have set up. The bank will only send if the buyer is 18+ or not to the store. So no identification data is send to the store not even the actual age.

Not necessarily the best approach but a widely and officially recognized one:

https://digital-strategy.ec.europa.eu/en/policies/eu-age-verification

You could do it with an offline AI model and Python. You download, firewall it, it checks your image and shits out signed text message of your approximate age range and the time.