You're going way too far in your reasoning, acting as if the state had to directly enforce it itself. But it doesn't have to: social networks are run by companies that makes profit doing so and that can be strong-armed into doing the control by themselves or be fined if they fail to comply (and we're talking about company whose entire business is about profiling their users to maximize their ads revenues, so they have zero difficulty recognizing teenagers, and more importantly content that is targeted at teenagers).
And even more importantly you're missing the point of why people go to social networks in the first place: because everybody they know is there! If it becomes cumbersome to access, most people won't go, and then there's no more appeal. It's not as if it was porn or stuff like that, that has a purpose on its own that makes people willing to circumvent the restrictions no matter what. Social networks are “networks” and if you break the network effect, you've broken the system. American people don't go on VK not because it's less good than Facebook, but because there's no point in doing so.
And even more importantly you're missing the point of why people go to social networks in the first place: because everybody they know is there! If it becomes cumbersome to access, most people won't go, and then there's no more appeal. It's not as if it was porn or stuff like that, that has a purpose on its own that makes people willing to circumvent the restrictions no matter what. Social networks are “networks” and if you break the network effect, you've broken the system. American people don't go on VK not because it's less good than Facebook, but because there's no point in doing so.