As the global population of people 65 and older is projected to triple to 1.5 billion by 2050, public concern about the graying of society varies widely across the world, with Americans much more optimistic than those in many other countries thanks to immigration, a new study finds.

That optimism is not unfounded: While one in five U.S. residents is expected to be 65 or older by 2050, the country is aging at a slower rate than other nations. This is largely due to the high rate of immigration, according to the report, “Attitudes About Aging: A Global Perspective,” released Thursday by the Pew Research Center.

With fertility rates dropping and longevity increasing, nations across the world will see their ranks of older people swell, with about one-sixth of the global population expected to be 65 or older by 2050. But perspectives about this diverge dramatically depending on a country’s demographic, economic and political context, according to the report.

In surveys in 21 countries last spring, 22,425 people were asked whether aging posed a problem for their country, whether they anticipated having an adequate standard of living in their old age, and whether governments or families and older people themselves should bear more responsibility for the elderly.

Americans ranked among the least worried, at 26 percent — a stark difference from residents of the United States’ main economic and political partners in Europe and Asia, where more than half the respondents said aging was a major problem.

The United States is projected to age more slowly, in large part because immigrants and their descendants are expected to shore up the ranks of young people of working age.

Immigration is projected to increase the U.S. population by 89 million by 2050, the report said, while the populations of Japan, China, South Korea, Germany, Russia, Italy and Spain are expected to stay the same or decrease.

Concern about the issue correlated closely with a country’s rate of aging, said Rakesh Kochhar, a senior researcher at Pew and the report’s lead author.

“In countries that are aging the most, people are indeed the most concerned,” he said. “Europeans are already old and continuing to get older. Younger countries tended to be more optimistic.”

The most worried respondents were in Japan, South Korea and China, where 87, 79 and 67 percent of people, respectively, said aging was a major problem for their country. Europeans also expressed a high level of concern.

But even as attitudes in the United States have become more negative since the 2007 recession, Americans remain far more optimistic, perhaps because the country’s demographic trajectory is projected to remain relatively stable.

“Unlike Europe, it’s going to maintain its demographic status, where its share of the global population is unchanged,” Kochhar said. As the United States increases in size relative to other countries whose populations are shrinking, he said, it is likely to see economic benefits.

Africa is expected to see the greatest population growth by 2050, with an increase of 1.4 billion people. Europe is expected to shrink by more than 30 million.

Respondents in the most rapidly aging countries expressed the most concern. Japan and South Korea are expected to have majority populations older than 50 by mid-century, and European countries already have some of the world’s oldest populations.

Latin America and Asia are projected to age the most rapidly, but Latin America has a relatively low median age, 10 years younger than that of the United States., so it is expected to be on par with it by 2050.

Confidence in one’s standard of living in old age was related to both the country’s rate of aging and its economic vitality, the report said. Respondents who believed the government bears more responsibility for taking care of older people tended to be less optimistic about their old age.

The coming demographic changes could significantly alter the distribution of global power in coming decades, with younger countries potentially able to pull ahead of older ones, the report said.

The older the population, the more people are past the age of contributing to economic development and the more people are dependent on government services and family caregiving. This is defined as the old-age dependency ratio, or the number of people older than 64 per 100 people age 15 to 64, or working-age.

In South Korea and China the old-age dependency ratio is expected to quadruple by 2050. In Spain, it will nearly triple; in Japan, Germany and Italy, it will double. These countries will have almost as many dependents as working-age people by 2050, the report said.

Countries where the younger populations are expected to increase, such as Egypt, Pakistan and Nigeria, have a brighter outlook, with more economic resources expected to be freed up for economic development.

In the United States, the swelling ranks of older people are expected to be offset by the infusion of more younger people, putting the country in a better position than many of its major economic and political partners, the report said.

But population estimates are only one factor when predicting a country’s economic viability, the report said.

People might choose to work longer in aging countries, boosting economic vitality. In the United States, for example, the original Social Security full retirement age of 65 has been edging up since 1983 and is expected to level off at 67.

Stability, social customs, governance and corruption also play into the equation, Kochhar said.

“Is demography your destiny? You can’t assume so,” he said. “It’s a combination of things. But demography does set the stage in positive and not so positive ways, and the rest is up to you.”