I had over 10000 food (about 6500 now) and most, if not all, were healthy when I left them. Five of the villagers were esteemed elders, and one of the youngest ones was an adept doctor. I thought the bones stayed until you opened/ran the game again. I've never 'lost' a villager before... I've had them die while not playing, but they're always accounted for on the tombstones.