Movie ratings are voluntary in the US. Restrictions on selling porn to minors are orthogonal to movie ratings. It's generally legal for a theatre to sell a ticket for a PG-13 movie intended for 15-year-olds to a 12-year-old who hasn't received parental permission. Most theatres would refuse to sell the ticket, and most parents wouldn't let their 12-year-old child go to a movie theatre alone, but the bulk of the responsibility of preventing children from watching inappropriate movies falls on parents and guardians.

Banning children from social media is like banning children from movie theatres. A ban should consider that (1) different restrictions are appropriate for different ages of children (e.g. 12 vs 15), (2) depending on the country (e.g. the US with the First Amendment to the Constitution), children may have information access rights that parents can take away but governments can't, and (3) children in unhealthy relationships with parents or guardians (e.g. transphobic/homophobic parents of LGBTQ+ kids) should be able to access some kinds of social media without letting their parents/guardians know.