I was going to post something similar, but you saved me the trouble. I've known exactly ONE exception to that rule and they were both pretty exceptional people to begin with. She was early 20's, had completed her degree and was working as an elementary school teacher. He was mid to late 30's with a few more years to completing a 20 year retirement from the military as a senior NCO.
And I would guess for the couple you speak of, that there was something in the way of a friendship at church or other setting — something there before the relationship takes a turn to...much, much, more.
I have seen that a few rare times too, but it is always a few years in the happening and does not start as a ‘dating’ situation.