Well, my personal set-up (not used for anything except a Sorting song, so far) is that Godric was the youngest, barely an adult when they started the school. Helga was an elderly widow who'd raised a houseful of children and had been Godric's tutor as a child in matters of magic, particularly Herbology and "practical magic". Rowena was a nun of indeterminate age who took care of the "book-learning" aspect of education. Salazar was part of the invading enemy force, their enfant terrible, about eight years older than Godric. Because Salazar was being used against his will as a child, he was allowed to live and went off to the fens, and little Godric, much to his guardians' annoyance, would visit him. When Godric came of age with his wacky idea for a school, he wanted his old teachers Helga and Rowena, and argued passionately to include Slytherin as well. (After all, the Sorting hat goes on about how Slytherin and Gryffindor were friends, as were Ravenclaw and Hufflepuff.) Thinking of them as at totally different phases of life--not something I ever thought about exactly; it's just how they came to me--is probably why the shipping never occurred to me! It would be like shipping Harry/Molly.

(If that's shipped, I don't want to know about it.)

Still, should have known.