> Parents buy those phones, phones could easily have a "user is a minor" setting (and a flag sent to all the sites that want one)

That's basically the California law! I hope other states adopt the "ask nicely for age but no online verification." OS setup asks for birthday, but you can just say January 1.

I also thought it should just send the flag, but I've heard there's good reasons not to. There's normally an affirmative prompt from the user to agree to send the age bucket data.