I believe he was in his fifties or even sixties when he decided he wanted to enter academia. He applied for a PhD program at that age, and had a hard time getting the powers that be to accept him. First, he was (of all things!) a science fiction writer! And he was way too old to enter an academic career!

Somehow he got permission, got his PhD, and entered advanced teaching. Whether he actually got to professor rank, I don't know.

Dave Oesterreich