I never thought about that and I find it might actually be the right way for a solution... I was just looking into the Google Play Store now: in Brazil apps need to have an age rating, facebook and instagram for example are 16+, but browsers like firefox, chrome, etc are rated for 'all ages', even though you can access the whole internet through them. If we actually did what you said and inverted the logic: restrict everything to 18+ and specifically mark what is allowed to under-18s it would make parental controls much more effective.