What's the opposite of the youth bulge
bubble? In developed countries like the U.S., it's a youth void, with otherwise dynamic cities becoming involuntary childfree zones as the overall birth rate drops and families leave expensive urban areas for less expensive--and energy-inefficient--suburbs. Also, the presence of children is a primary driver for parks, trails, and public safety--factors that that play into greening cities. So it's worth keeping on the Worldchanging radar.
And so is how the issues get framed. According to this this New York Times article by Timothy Egan, improvements to the light rail system in Portland, Oregon--a place I always mention when talking about sustainable cities--have contributed to the rising housing costs that are driving away young families. He fails to mention the crazy real estate speculation of the early 1990s, which drove urban housing prices beyond the means of many still living on an Oregon median income long before any new work on Portland's light rail began, and helped push low- and moderate-income folks of all sorts out of the centers of Oregon's cities. It's inaccurate to frame the issue as good mass transit (key to livable and sustainable cities) versus kids and elementary schools.