I speak from the middle days - my father became a cutter in something like 1927. He worked his way up in a few studios and some of his peers were famous later as directors - you would recognize them; he had his own ball of wax to deal with - with the army air force and the studios later. I was a natural for all that, though there were few women at the time (I might recognize the names, just can't name them) and decided I was interested in medicine instead. Right around my early teens I was interested both in editing and advertising, and then read Vance Packard at the same time I was reading about the history of medicine.
I've heard there is a kind of closed system for getting in as a film editor, but I don't know that personally.. but learning editing is (I think) a route in and valuable for a director, whether out of some school or working by her or his self.
I can't speak to it now, though I still am friends with at least one good editor and a director. I don't ask them this stuff, more usual friend talk.