The survey by the Pew Research Centre covers 21 nations. (Agencies)
Changes in the world's demographics "could alter the distribution of global economic power over the coming decades," Rakesh Kochhar, senior Pew researcher, said on Thursday.
"Demographically, at least, America is poised to maintain its global status while many European and East Asian nations shrink either in absolute or relative terms. India and several African nations may benefit from the projected demographic trends," Kochhar said.
Just 26 per cent of Americans said ageing in their country is a "major problem," according to the report released on Thursday.
Concern about ageing was highest in Asia 87 per cent in Japan, 79 per cent in South Korea and 67 per cent in China.
In Europe, more than half of those surveyed in Germany and Spain pointed to increases in the older population as a major problem in their countries.
Americans were relatively optimistic that they will have an adequate standard of living in their old age. About 63 per cent of those surveyed said they were "very confident" or "somewhat confident" about their retirement security.
They ranked near the top along with other countries with younger populations or strong economies: China, Brazil, Nigeria, South Africa, Kenya and Pakistan.
According to the Pew survey, retirement security confidence was lowest in Japan, Italy and Russia.
Because of high rates of immigration, America has one of the higher birth rates in the developed world. Its population is expected to rise by 89 million, to 401 million, from 2010 to 2050, with immigrants and their descendants accounting for about 80 per cent of the increase.
China, currently the most populous nation at 1.4 billion, is projected to add only 25 million people by 2050. Global ageing is driven by social and economic factors.
Compared to previous decades, marriage rates in general have fallen and birth rates have dropped, due in part to the development of contraceptives.
Also playing a role in declining births are the rising costs of raising children, increases in women who attend college and are in the labour force as well as the reduced need for children to care for parents in old age.
Globally, the median age will rise from 29 to 36 by midcentury. That's faster than the US increase from 37 to 41.
The US, South Korea, Germany and Britain were the only countries surveyed in which more than a third of respondents said the elderly bear the most responsibility for their own well-being, as opposed to government or their families.
Widespread ageing will have a financial impact. Public pension spending as a share of the gross domestic product is expected to rise in all surveyed countries except Mexico and India from now until 2050, the report said.
The survey by the Pew Research Centre covers 21 nations.