Mostly it's enough to have "if a minor gets caught, he will be punished". Similar with the alcohol, except that alcohol consumption is short in duration, while usage of social media is constant, making it much more likely to be caught.

In this kind of case, the liability should clearly start before 18. Solicitations/attempts should be punishable as well. Punishment should also be "retroactive" - "oh, you are 20, but have been using this account for 10 years? Smack!" with consistency and public publicity/propaganda.

A much greater punishment should be done towards the adult who allowed it.

In a similar way as buying alcohol or drugs for minors.