I know what you're thinking, and I agree, so let's just get this out of the way: "Having someone to take care of you when you get old" is a terrible reason to bear children. But since you may currently be in the trenches of wiping the bottom of a person who keeps calling you Squishy Belly Mummy and wondering, "So when is this whole parenting thing supposed to pay off again," let's look at some numbers, shall we?