Villagers tend to live to advanced ages for some reason, when they live their lives while you're away - the game's behavior when you're away is different, because the game calculates what should happen during that time when you load up the game. You sometimes have things like impossible ages, a lot of births at exactly the same time, and other oddities crop up after a while away from the game.
Playing on fast only gives the game more Isola-time to act weird, since according to the villager's clock, you're away twice as long.