[Rosie] I know that sounds unlikely, but remember that at the time of conception he was probably only 75. On a different note, can I add one more note of both sympathy and admiration for what you are doing. I freely admit to relief at having been spared that ordeal with either of my parents: my mother died fairly rapidly (and as these things go, relatively painlessly) of cancer, and my father of a heart attack just as it was starting to seem that he was going to need full-time care. I witnessed the alternative -- at a distance -- with both of my grandmothers, and it is obviously very difficult for everyone involved.
[Rosie] When I was at school, there was a boy there whose father was in his 80s. (I know it sounds like one of those silly school rumours, but you'll have to trust me that this one was true. Apparently the dad had plenty of money and the rest is history.)
[Rosie] Yep, your maths is right. It will perhaps ease your incredulity if I note that my grandmother was much younger than my grandfather (to the tune of just under fifty years, b. 1917).