Preventing teenagers from doing things that adults can do is pretty normal. They can’t drink, smoke, or drive until they’re old enough - at which point we decide it’s their life.
To some extent, if adults want to drink from the firehouse of poison we let them.
To other extents, we section those who inflict harm upon themselves or are a danger to others. Our social norms are fairly inconsistent on this and it's easy to make a case (though not widely accepted) that the sort of maladaptive social media use by older adults is both a form of psychic self harm and a danger to society.
To some extent, if adults want to drink from the firehouse of poison we let them.