The world's populations are aging rapidly, triggering demographic changes that will have a profound impact on economies, government expenditures and international migration patterns. In the past century, life expectancy has doubled, while the average family size has shrunk. By 2050, the number of children under 5 is expected to drop by 49 million, while the number of adults over 60 will skyrocket -- by 1.2 billion. An unprecedented number of senior citizens will be depending on diminishing numbers of younger workers to contribute to pension and health care programs for the elderly. And it's not just a problem for wealthy countries: Developing countries' elderly populations are growing faster than in the developed world. For example, in 20 years, China will have 167 million senior citizens -- more than half the current U.S. population. On the positive side, some demographers believe aging societies will be more peaceful, since seniors suffer fewer crime and drug-abuse problems. And with fewer children, there could be more money per capita for their education.

Alan Greenblatt has been writing about politics and government in Washington and the states for nearly two decades. As a reporter for Congressional Quarterly, he won the National Press Club’s Sandy Hume award for political journalism. While on staff at Governing magazine, he covered many issues of concern to state and local governments, such as budgets,taxes, and higher education. Along the way, he has written about politics and culture for numerous other outlets, including the New York Times, the Washington Post, and NPR.org.