Control over who they can talk to (if needed), certainly monitoring of both who they talk to and in many situations what the contents are

At some point between the age of 0 and 18 the child has to be fully ready for an independent world. A cliff edge is a terrible idea, allowing 3 year olds unmonitored uncontrolled conversations with strangers is a terrible idea, but not allowing 15 year olds to talk to their friends is a terrible idea.