Meta == Phillip Morris - This is a public health issue and will likely need to be treated like tobacco. Kids can't vote so I don't see the political motivation.
The solution, however, isn't prohibition or age restrictions; it's either regulating the algorithms or holding these companies responsible for the adverse outcomes their platforms contribute to. Safe harbor laws made sense when tech wasn't filtering/promoting content, now that they are influencing the material we see, these laws must no longer apply.
This may mean adopting a modern equivalent to libel laws. Something akin to: if an algorithm pushes false information, the company behind the algorithm can be sued for harm. Disallow terms of service that force arbitration or cap liability limits.
When I was 18 the legal drinking age had just been reduced to 18. That only lasted a couple of years. I don't think I'd vote for lowering it to that age again actually.
It really isn't a bad thing for kids to be told they can't do something occasionally. It kind of helps prepare them for being an adult where it happens all the time.
So what you are describing is just the base case.