For kids under say, 14 years old, you can simply look at children in your direct vicinity and observe the impact of abundant use of things like Youtube, Roblox and others (sorry I'm not hip with it), where it is impacting their motor and cooperative skills, as well as their confidence.
Where is are your sources for this?
Why did you throw Roblox in there?
Video games have been shown with abundant evidence to benefit hand eye coordination..?
So yea, a ban is definitely a ham-fisted approach, but in my opinion, given the already sketchy situation surrounding the privacy for such an age check, I personally can't think of a better solution or middle ground to keep the positives of social media. Maybe force the platforms to abandon their current algorithms?
So because of your failure to imagine a solution, kids should be barred from all of it? The solution is take the money out of social media (like here) and have MORE human adult moderators in online communities who have a vested interest in clamping down on toxic people, not lock kids in a box in a way that destroys all of our freedom including theirs.