Of course this is technically unenforceable. There will always fbe workarounds. You could smoke as an 11 year old if you were determined enough.
But we need more pushback and dialogue on social media’s role in the common discourse. For a while, nobody talked about smoking being bad and it became normalized while killing a lot of people. Seatbelts. Dumping waste in the river… most people go with the flow of common consensus until that consensus is scrutinized.
Even more if the system worked in a centralized way: this email/phone is used exclusively by a kid, so now all on-boarded companies must delete their accounts and not allow them to register again.
If you're going to introduce legislation like this, then it needs to include provisions that it will not permanently bar those users access to the services once they are of age. I manage my children's social media interaction (near 0 with the exception of YouTube), if their accounts get permanently disabled that would be unfortunate in the future when they are old enough.