What they say to kids who want pets: Are you sure you're not just saying you want one because all your friends have one? Remember, it's not going to be small and cute forever, it will grow up eventually! It's a living being that will depend entirely on you for the rest of its life. Are you really sure you're ready for this?
What they say to adults who DON'T want kids: Oh, you'll want one sooner or later. Everybody does, after all. Besides, babies are soooo cute, aren't they? You'd better hurry up before you get too old!