That is not necessary for age verification. You just have to go through a working verification process and then you just get the checkmark in the database. most sites that really verify your age use credit cards for that, which are of course sub par, because children can just take the cards from their parents, but it is better than nothing
According to this wikipedia article it is mandated for pornography in Germany and the UK. Interestingly enough some alcohol companies in the US also try to verify their users age to ship them alcohol… So yeah a pretty isolated problem I guess