Cities have been in ascendancy for the last few decades—the past several years in particular—and a major reason for their success has been their appeal to Millennials (those born around 1980-2000) and the "creative class." As a result, cities have acquired a reputation as being the place that young people go after they finish college to live in small, expensive apartments, party all the time, pay too much for coffee, and generally delay adulthood as long as possible. Everyone grows up eventually though, so the influx of Millennials has a lot of people worried—some sincerely, others not so much—that when these kids finally grow up and decide to have children of their own, the whole urban experiment's going to blow up in our faces and cities are going to revert to the bad old days of the crime-ridden, blight-filled '80s.

But we should stop worrying. Some young families are going to leave the city, and not only is it not going to be as bad for cities as people think, it's probably going to be great for America. Cities are already the most productive places on the planet and the primary exporters of intellectual goods; why not embrace the export of urbanism, too?

One reason I'm not too concerned about the exodus of Millennials has already been covered pretty thoroughly, and it's also generational: baby boomers are hitting retirement age, and without any kids at home and with less physical mobility they're seeing the appeal of living in smaller homes, closer to their daily needs. Even as many Millennials leave cities, the largest generation in history is getting older and the challenges of keeping up a 3,000 square foot home and driving everywhere will weigh ever-heavier.