“Age 70 is the new 60,” proclaim authors at the United Nations Population Fund. This isn’t a new fashion publication or even specifically a comment on life expectancy. This is the U.N. authors’ reason to not worry about the record-low birth rates in the developed world.

It’s a popular counterargument against those who worry that a lack of young people, and thus an aging population, could sap economic productivity: We can make up for the lack of working-age people by all working a lot harder and a lot longer than we used to.