My assumption (made admittedly with no evidence whatsoever) that you believe it's possible for someone to verify their age without giving up any anonymity or privacy, even temporarily.

one of the reasonable solutions i allude to is an over/under 18 age attestation at time of account creation on your OS. websites say "18+ content" and the OS either displays or doesn't display the content. there is ~0 privacy or anonymity concerns as 3rd parties don't get any communication about whether the content was shown.