It’s like the last twist in a horror movie. You think the baby boomers are dead, or at least easing into retirement, and then they thrust their claw up from the dirt, or lunge from the bathtub with a knife. Or take over a presidential election. Or decide to “liberate” your country from an economic partnership that is imperfect but deeply popular among younger generations whose vision is more kaleidoscopic than the color scheme of a national flag. “YOU STOLE OUR FUTURE FROM US,” said a blue banner hoisted by two young women in front of Big Ben after the...