It was already de-facto illegal for people under 13 to have social accounts. Do you know any kids younger than that on social media? I do. This is the equivalent of paper bags for beer cans - it will do nothing to fix the problem, but it will make children slightly more secretive about their internet usage, which will make some situations (grooming, bullying) even worse.
That isn't really how laws work. You don't say: "Driving under the influence is illegal, but I totally know people who drink and drive so the law is useless".
A solution doesn't need to be 100% effective for it to be hugely beneficial to society.