I started hearing it about two years ago, and then I seemed to hear it constantly: people in their late teens and early twenties complaining, quite sincerely, that they felt old. The first case I remember was a college student at the University of Nevada, Las Vegas. I’d just finished speaking to her class about an essay I’d written on electronic surveillance, and during the discussion afterward she grumbled that kids a few years younger than she was — kids who’d been using smartphones and social media since their grade-school days — seemed to have no concept of personal privacy. Her…