Blame the '60s for America's perpetual adolescence? Nah, blame the '20s. In the 1990s, most people who played video games were teenagers. Now the average gamester age is nearly 30. Cultural products aimed at tots and preteens capture the attention of adults. "SpongeBob SquarePants," intended for the 6-to-11 age group, draws almost 19 million viewers from the 18-to-49 crowd. Some famous museums, uncomfortable with their adult role as guardians of historical memory, have gone adolescent, staging exhibits on motorcycles, hip-hop and "Star Wars" movies. Many college courses, even on major campuses, make rainy-day activities at summer camp seem profound. Such...