The problem at the moment is that the technology does not aid parents in this.
Personally, I would like to see the existence of this sort of age-gating API be mandatory, and set some government guidelines, but leave it up to parents whether or not they wish to use it. Because right now, unless they are hovering over the shoulder of their children every moment they're on a computer, there's literally nothing they can do with available technology to prevent children accessing age-inappropriate material. So a law that can help them out without forcing their hand would be great.