I would argue that it is the responsibility of parents to decide how to do that. But if you want a legislative solution, there already is one (COPPA) which prevents people under the age of 13 from joining social media.

If you think those legal protections aren’t fit for purpose (they were created long before social media even existed), then you should take that up with your legislators. I personally wouldn’t trust them to approach that task without implementing something horribly tyrannical, like implementing a requirement for a full KYC process for creating social media accounts. So I’d advise that you be careful what you ask for in that respect.