I love the sentiment and would love to see this working, but lets be realistic, It never will how things are now. Just a few things that comes to mind:
Companies and public services use social media sites for customer service.
Social media corporations are almost never held responsible for propaganda and any of the sick stuff they serve up on silver platter. Putting up an age warning Y/N window does nothing. They will still target 15 years or younger audiences.
People profit off other peoples data and corpos will fight hard not to loose a huge chunk of this money machine.
Social media sites only goal is to show you content and make you go through as many ads as possible. They will definetly not consider people under 15 and change their algorithms to make their product less appealing.
Parents are also addicted to social media, like what are the chances a kid of two chain smokers will also try cigarettes?