There are ways, like double blind age verification, in which neither the website knows anything other than "yes, >18", nor the verificator knows anything other than "I was asked if user X is >18, checked, yes". Website doesn't know actual age, verificator doesn't know which website it is or for what action was the request performed.

In fact it's even in the EU Commission's official guidance on how it should be done : https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=OJ:C... (point 46).