Over the next couple of decades, the smallness of Generation Z will lead to tightness in the labor market, argues Levanon. He bases this on a metric called the 17-to-64 gap. It's the idea that the labor force grows when the number of 17-year- olds (who are about to turn 18 and enter prime working age) exceeds the number of 64-year-olds (who are about to turn 65 and exit the prime working age cohort).

As recently as 2000 there were still 2 million more 17- year-olds than 64-year-olds. The gap shrank to around 700,000 in 2011 as the oldest boomers hit age 65 and it will shrink to almost nothing by the mid-2020's, Levanon says, citing Census Bureau projections.

"The contribution to population growth of this gap is about 0.4 [percentage point] now, and it will gradually shrink to almost zero in the next decade," Levanon and his colleague Michael Paterra wrote in a blog post last month.

The growth in America's working-age population in the 2020s will be sustained primarily by immigration, Levanon and Paterra write. And it won't be much: a measly 0.2 percent or so in 2025, down from around 1.3 percent in 2000.

These numbers may sound surprising, but they aren't exactly pulled from thin air. They're based on Census Bureau projections. In a new report, the Census Bureau projects that the working-age share of the population will fall from 62 percent in 2014 to 58 percent in 2030, then level off.

Of course, this is only the supply side of the equation. Whether the dearth of Gen Zers results in labor shortages depends on the demand side--namely, how much labor the economy will need in coming years. Some economists argue that computerization, robots, and foreign outsourcing will kill demand for workers.

Others say that those forces will simply free up American workers for jobs that can't yet be imagined yet, as happened during the transition from farm to factory and later from factory to the service economy.

If labor shortages do develop, one force that could ameliorate the problem is the rise of what a new report from McKinsey Global Institute calls "online talent platforms"-- websites like LinkedIn, Glassdoor, and Monster that match people with traditional jobs, plus online marketplaces like TaskRabbit and Uber that connect people to contingent or freelance work. Worldwide, "Up to 540 million individuals could benefit from online talent platforms by 2025," the institute says.

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