The website operators in each region could look up what is the age of consent for the GeoIP browsing them. Here is one for parts of the world [2].

Or they could just add the header to anything that is potentially adult which is what I am proposing. It would be up to the parent to decide what is appropriate for their small children to view. Parents should already know what is legal for their kids to view.

[1] - https://en.wikipedia.org/wiki/Age_of_consent_by_country