Many might’ve seen the Australian ban of social media for <16 y.o with no idea of how to implement it. There have been mentions of “double blind age verification”, but I can’t find any information on it.
Out of curiosity, how would you implement this with privacy in mind if you really had to?
Frankly, the only sane option is an “Are you over the age of (whatever is necessary) and willing to view potentially disturbing adult content?” style confirmation.
Anything else is going to become problematic/abusive sooner or later.