People may talk about generations as though they proceed in some orderly parade, but it’s really more like a tug of war: Whichever age group outnumbers the others gets to pull an entire society deep into its own habits, neuroses and preoccupations. As a result, one of the best ways to understand popular culture is simply to consult a chart tracking the number of Americans born each year. Most prominent will be the huge swell of people born after World War II, who have dominated the national psyche for as long as any living person can recall. Then comes a 1970s trough, a tiny cohort of poor souls who will never dominate anything and are best known for being sardonic about it. Then comes a rally, and another peak: American adults, at the moment, have a pronounced tendency to have been born around 1990. A lot of our cultural noise these days is just the sound of a nation’s center of gravity shifting, all at once, across four entire decades — and landing on a group of people who, whether they realize it or not, can now manhandle the world the same way their elders did.