The joke itself, if it even IS a joke, is /at least/ 25 years old, almost older than the mainstream internet, because I remember first hearing it while I was taking driver's ed to get my license. And the version I heard said teen pregnancies drop drastically after age /25/, lol.
And I say "IF it is a joke" because given the things some people spend money to do studies on, I don't find it all that hard to believe someone could come out with a study like that.