“The Curious Case of Benjamin Button,” which occupies around 25 pages in the collected works of F. Scott Fitzgerald, is a slender piece of whimsy, a charming fantasy about a man who ages in reverse, descending through the years from newborn senescence to terminal infancy. As Fitzgerald unravels it, Benjamin’s story serves as the pretext for some amusing, fairly superficial observations about child rearing, undergraduate behavior and courtship in the late 19th and early 20th centuries.