Men also die earlier than women so it's possible most students grandfathers are already dead before they reach college. I know this was the case for me.
For me as well. And I have three grandmothers, courtesy of my stepmother. Two of them died while I was in college, the third is 97 y.o., and I've got two more semesters to go, so there's a chance I may be one of those people who dips into the "negative grandmothers" category.
This is a very good point. Both my grandfathers passed away when I was young (i.e. before I took any important exams), whereas both my grandmothers are still alive and kicking.