But it does matter, and if the trend persists, it could mean lower living standards for most Americans in the future.

One of the great strengths of the U.S. economy, especially compared to Europe and Japan, is a relatively high birth rate, which keep the population young, on average, and population growth robust. "Everybody comes into world with one mouth and two hands," says economist Donald Boudreaux of George Mason University. "It's generally true that most people produce more than they consume." ...