AT no point in recorded history has our world been so demographically lopsided, with old people concentrated in rich countries and the young in not-so-rich countries.

Much has been made of the challenges of aging societies. But it’s the youth bulge that stands to put greater pressure on the global economy, sow political unrest, spur mass migration and have profound consequences for everything from marriage to Internet access to the growth of cities.

The parable of our time might well be: Mind your young, or they will trouble you in your old age.

A fourth of humanity is now young (ages 10 to 24). The vast majority live in the developing world, according to the United Nations Population Fund.