Ever since the global financial crisis, economists have groped for reasons to explain why growth in the U.S. and abroad has repeatedly disappointed, citing everything from fiscal austerity to the euro meltdown. They are now coming to realize that one of the stiffest headwinds is also one of the hardest to overcome: demographics.

Next year, the world’s advanced economies will reach a critical milestone. For the first time since 1950, their combined working-age population will decline, according to United Nations projections, and by 2050 it will shrink 5%. The ranks of workers will also fall in key emerging markets, such as China and Russia. At the same time the share of these countries’ population over 65 will skyrocket.

Previous generations fretted about the world having too many people. Today’s problem is too few.

This reflects two long-established trends: lengthening lifespans and declining fertility. Yet many of the economic consequences are only now apparent. Simply put, companies are running out of workers, customers or both. In either case, economic growth suffers. As a population ages, what people buy also changes, shifting more demand toward services such as health care and away from durable goods such as cars.

Demographics help explain why a historically weak recovery in the U.S. has nonetheless seen the unemployment rate drop by half. The economy doesn’t need as many new jobs to employ the smaller net flow of entrants into the workforce. For example, home builders are simultaneously suffering from shrinking demand since the homeownership rate is declining, and from labor shortages as the baby boomers retire.