So what's your point? That cars don't need a seatbelt because we've banned kids from driving? Actually what you're doing is just proving my point. For starters that ban doesn't stop 16 and under from driving, it's just something that will be used to punish someone after it has happened. At best it's a deterrent because law-abiding citizens (including children) are less likely to break the law but I've personally seen kids take a car for a joyride and crash into someones yard so the ban isn't going to stop anyone who really wants to go and drive. And more importantly it doesn't really address the core problem which are collisions and crashes. Despite the ban adult people still end up in crashes. What does help are all sorts of regulations that car manufacturers have to adhere to make sure that the damage is minimized. Instead of having your brains splattered all across the road you might end up with a concussion instead.

And the same applies to social media. Banning kids from social media isn't going to do anything about the addiction or mental health problems social media causes to people. It won't even stop kids from using social media because kids are smart and if they want they will find a way around the ban. Now I'll compromise a bit and say that banning kids from social media isn't the worst idea because kids are more impressionable and social media might have a much bigger effect on a young mind. But if the goal is to improve the mental health of kids you also have to target the social media companies and regulate them to make sure their impact on mental health is minimized. Not only would that help kids but that would also help adults.