It could be part of the Accept header which already indicates language. It could serve as a signal that you only want to see content that is appropriate for tha

It could be part of the Accept header which already indicates language. It could serve as a signal that you only want to see content that is appropriate for that age bracket. Where appropriate means what the site wants to serve according to the laws of their jurisdiction. But to require OS to implement all of this goes way to far and together with the proposed verification mechanisms is a privacy nightmare.