Since the dawn of humans, we have faced one inexorable challenge — how to support the rise and — in the last half century or so — explosion of the population. But, in a momentous reversal, that age-old challenge is changing: the population of most countries is shrinking — for many of them at an alarming pace — and at the same time aging.

Much of the world teeters on the cusp of a childless, elderly future.

Why it matters: A growing, youthful population is typically a bedrock sign of vitality. In the industrial age, that’s included a growing economy, greater opportunity, advancing technology, and a more comfortable retirement for older people. The turnaround on all continents except Africa means supporting an increasing number of retired people with many fewer workers, and confronts the world with two primary solutions, both of them controversial.

• Loosen up currently fraught politics around migration from still-growing countries.
• Or populate our countries with robot helpers.

Whichever the case, almost no one seems prepared. People are going to have to work longer, experts say. And the […]