Why just teens though? Getting manipulated by algorithms crafted to maximize screentime and ad revenue is bad for anyone.
These platforms rely on ads to survive. Which means it should be easy to regulate them. You can prevent them from selling ads at which point they will be forced to comply. If they don't, someone else will get the ad revenue. Europe is already hostile towards american tech giants anyways.
The possibilities are endless. Pass a law that forces all social media with more than x users to not implement constant scrolling, make their ranking algorithm open source, allow people to use their own algorithms, employ robust moderation etc.
Instead we have a blanket ban that requires id checks but leaves the manipulation machine intact so it can prey on adults. Mental health is not the real issue here. They want to be able to track people and destroy anonimity online. Children are a convenient excuse.