In 1984, the comic-book artists Kevin Eastman and Peter Laird created Teenage Mutant Ninja Turtles, a dark and snarky satire starring the cartoonish crime fighters. Then, in 1987, Eastman and Laird licensed their characters to a toy company, and shortly after, a bright and colorful children’s animated series emerged. Since then, the Turtles brand has morphed and evolved in various directions, with many a reboot along the way. Movies, TV shows, comics, video games, and whole universes of toys have kept the characters fresh for each subsequent generation of children. But the latest feature film, Teenage Mutant Ninja Turtles: Mutant Mayhem, a surprise hit this summer, is the first entry in the franchise to successfully merge both strands of Turtledom, a creatively engaging animated movie aimed at kids that bubbles with the punkier energy of yore. Its success highlights something sort of unbelievable: A concept centered on wisecracking ninja reptiles has been viable for nearly 40 years. Surely nobody would’ve predicted that when it launched. The Teenage Mutant Ninja Turtles were introduced as four turtles …