Advertisers are famously obsessed with young people, and so television, by necessity, is obsessed as well. In recent years, network brass have been at a loss to explain where all their young viewers have gone. Some say they went to cable, some say they went to the internet, and some say it shouldn’t matter, anyway. But what if the 18-to-34s haven’t deserted TV after all? What if they’ve just moved their “prime time” back a few hours? A close look at demographic ratings shows that young people are watching plenty of TV: they’re just turning on the set a lot later.