That's much harder to implement. If you ban advertising eg. online casinos to underge users, the service would have to send two different ads (one 18+ and one for younger people too), and the browser would have to decide which one to play. Same for eg searching on google, browsers would have to filter out every <div> with adult content, meaning half the page would be empty, but with a browser "underage" flag, google could just turn on safe search and not allow turning it off.