OVER THE next few decades, the developed countries will age and weaken. Meanwhile, dramatic demographic trends in developing nations—from resurgent youth booms in the Muslim world to premature aging in China and population implosion in Russia—will give rise to dangerous new security threats. Some argue that global demographic trends are progressively pushing the world toward greater peace and prosperity. They are wrong. The risks of both chaotic state collapse and neoauthoritarian reaction are rising.

Everyone knows that the developed world is aging rapidly. Graying workforces will become less flexible, less mobile and less innovative; rates of savings and investment will decline; current-account balances will turn negative and foreign indebtedness will grow. Rising pension and health-care costs will place intense pressure on government budgets, crowding out spending on defense and international affairs; militaries will face growing manpower shortages.

Although it is less widely appreciated, the developing world is also aging. Like the developed world before it, the developing world is now in the midst of what demographers call the “demographic transition”—the shift from high mortality and high fertility to low mortality and low fertility that all societies undergo in the course of development. Since 1970, the average fertility rate in the developing world has fallen from 5.1 to 2.9. Meanwhile, the median age of the developing world has risen from 20 in 1970 to 26 in 2005. It is projected to keep rising to 31 in 2030 and 35 in 2050—at which point the typical developing country will be about as old as the United States is today.