There is a looming catastrophe stalking the developed world. It promises to devastate the global economy, overwhelm hospitals, and decimate armed forces. What is the calamity that promises such misfortune? Not a killer virus, deadly terrorist attack, or natural disaster. It’s the aging of the world’s baby boomers, the coming tidal wave of senior citizens who will live longer, consume more, and produce less, seriously challenging societies’ ability to care for their graying ranks.

At least that’s how the dire warnings generally sound. Alarming forecasts bombard us about an impending demographic crisis in the United States, Europe, Japan, and even China that will reshape the way we live and work. In just two decades, we’re told, there will be more Americans who are older than 65 than younger than 15. By 2040, at least 45 percent of the populations of Spain and Italy will be 60 years or older. That same year, China will have 400 million elderly citizens. And in Japan, which is aging faster than any other country, more than 40 percent of the population will be elderly by the middle of this century. The fiscal burden of supporting this rapidly expanding segment of the global population not only threatens to bankrupt national healthcare systems and shrink armies as countries’ median ages run past military age, but also revolutionize electoral politics, with political clashes no longer governed by right versus left, but young versus old.

If it sounds distressing, it shouldn’t. The gloomy projections are deeply flawed. The reason lies in the misleading way in which we measure age. Typically, a person’s age has been determined by the number of years since his or her birth. We are so accustomed to measuring age this way that most of us have never given it a second...