Two converging trends are leading to some ironic demographic conclusions: first, the oft repeated truism that Millennials are flocking to urban cores, part of a process called gentrification; and second, that this urban lifestyle of job switching, meeting friends, potential partners, entertainment, and more is producing what the latest Canadian census describes as shrinking families. Fertility rates have declined from 2.7 in 1961 to 1.9 in 2011. The next big question, according to Rolf Pendall at The Atlantic, is "will Millennials stay?"—and if they do, will they raise families? If they don't, the ironic demographic conclusion will be that Millennials may yet save some cities, and destroy the country.

The so-called delay of adulthood is now the subject of a great deal of nervous hand wringing amongst media elites. Millennials, generally adults between 20 to 34 years of age, also known as Generation Y or the Echo Boom, have statistically delayed childrearing, postponed marriage, and opted out of building households. Opinions range on the circumstances of these decisions, including cultural and economic necessity. To be sure, establishment boomers fretting the collapse of political, social, and cultural institutions—like marriage—is a bit rich. The mountainous debts accrued by boomer entitlement, and inherited by their children, would make any clear thinking person balk at an "initiation into adulthood." It feels more like a hazing.

But the decline of marriage and family, especially, is not simply a cathartic thumbing of the nose at boomer sentimentality. Boomers, after all, blazed the trail for easy-peasy attitudes on marriage. Boomers gifted the data so that people like Charles Murray could argue in Coming Apart that positive family relations are one of the most significant stabilizing social factors. Stable marriages and healthy families are among the most powerful indicators of economic and social success in children.

So said the Institute for Marriage and Family in their own press release, responding yesterday to the new numbers. IMFC's Andrea Mrozek argued that there is a direct causal link between married parent families and poverty rates.

Pause the moral question, and even free-market libertarians have reasons to get nervous about the numbers. The demographic crunch, detailed by Cardus in much of its charitable research, points to the now famous inverted population pyramid, with a far higher number of aged Canadians needing the support of a shrinking number of the young.

Old news; but the common logic of Millennial urban revitalization concomitant with demographic collapse shows the double edge. Urban living and its many, now repetitively extolled virtues, appeals to Millennials by the same logic that they are delaying "adulthood." Cosmopolitan apartments and condominiums, public transit, and bicycle-renting appeal to the mobile, childless entertainment seeker.

Urban Institute estimates that Millennials will establish between 15 and 18 million new households between 2010 and 2020. They argues, "Millennials may affect tomorrow's cities as much as Baby Boomers have shaped today's suburbia." But the question, really, is whether they'll have families there. Because if they don't, that shaping will be one more short lived burst of attention-deficit enthusiasm.