At what age should children first leave the house on their own, visit their friend next door on their own, go to the cinema on their own, hitchhike from coast to coast on their own, etc.?

In a relationship characterised by consent, on those occasions when the other person is warning us that our proposed course of action may be unwise, and explaining why, we have every reason to trust that such warnings are not attempts to thwart us and ruin our fun, but are actually important—that it is actually in our best interests to heed the warnings.