America's future is looking gray.

Gray-haired, that is. The over-65 crowd is now the largest — and fastest growing — segment of the population, according to the U.S. Census. Birthrates, in the meantime, have hit record lows.

It's a scenario that's playing out across the globe, David Brooks pointed out in a recent op-ed article for The New York Times. Half the world's population lives in countries where there aren't enough babies being born to keep the population steady. During the past 30 years, the average number of children per woman in Oman dropped by 5.6. In Italy, where the birthrate is just 1.4 babies per woman, government leaders have experimented with the idea of paying people to have more children.

"For decades, people took dynamism and economic growth for granted and saw population growth as a problem," Brooks wrote. "Now we’ve gone to the other extreme, and it’s clear that young people are the scarce resource."

While the United States still boasts a higher birthrate than any other industrialized nation, the country's youthfulness can largely be attributed to immigration, according to Ron McChesney and Greg Overberg, geography and planning experts from Three Scale Strategy and Research in Columbus, Ohio. And now, that, too, is slowing to a trickle.

In an article published recently in New Geography, McChesney and Overberg estimated slowing immigration levels and faltering fertility, together, may propel the nation toward zero population growth within the next 20 years.

"We could see a possible leveling off of population much sooner and at a lower rate than forecasted, say around 360 million by mid-century instead of 478 million by the year 2100 as forecast by the United Nations Population Division," they wrote.

The demographic change will have implications for government fiscal policy and generate debate about government policy in encouraging births.

"Population growth has been a relative advantage for the United States and remains so," they wrote, "but we may have to consider whether this trend is inexorable."