Old people are taking over—and not just in well-known grey zones like Japan and South Korea. By 2050, one in every three or so Spaniards, Italians and Germans will be 65 and older (pdf, p.6), according to Pew Research Center.

But relax, Americans. Though by 2050, 20% of you will be age 65 and older—a big jump from the current 13.1%—the US will be in better shape than many countries, according to Pew data. Not only is the US aging much less rapidly than those countries; its population is also growing. In fact, by 2050, the US population will be nearly 30% bigger, says Pew. That combination will help keep the engine of the US economy churning long after other major economies start sputtering.

What’s the US’s secret weapon? The same thing it has always been: immigration.

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Immigrants tend to have more children than non-immigrant women, which keeps the country’s population relatively young.

From 1960 to 2005, immigrants and their descendants contributed more than half of the total increase in the US population. And from 2005 to 2050, they’ll account for 82% of that growth. Along with people simply living longer than ever before, that will add 89 million more Americans to the country by 2050—which is far better than the lot of most developed countries.

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Despite living in a country with more immigrants than any other nation—42.8 million as of 2010—many Americans still believe that immigration is a bad thing. Some worry that immigrants will steal jobs from native workers, drive down wages and soak up public spending. But immigration helps stoke economic growth.

The US should count itself lucky. For many countries, it’s too late to change immigration policies enough to slow the rate of aging or the shrinking of its population. Germany, for example, would need to admit twice the number of immigrants each year until 2050 to prevent its population from declining, says Pew. Russia would have to up its intake by a factor of four.

If the US admitted no immigrants between 2005 and 2050, its population would grow by only 8.5%, near the growth rate of Spain’s population. Surely, no American wants that.