"Email-based age assurance" has been deemed to be (capable of being) "highly effective" under the UK's #OnlineSafetyAct
Annoyingly, this means data sharing and only works if someone uses the same email address with multiple third parties.
My email address is well above the age of majority in the UK, but surely "Neil likes to reply in-line and signs email with GPG" is a good enough indication of grey-beardedness anyway.