The United States benefits from a traditional pro-immigration policy. Immigrants tend to be young in general and help to keep the US population younger. According to PRB data, between 2010 and 2017, about 7 million more people moved into the United States than left.

As the third-most populous country after China and India, the United States is ranked the 34th oldest with 15 percent of the population aged 65 and older in 2018. Its aging rank is projected to decline to 56th in 2050 at 22 percent.

The most populous country, China, is younger than the United States with 11 percent of the population aged 65 and older in 2018. However, China is projected to be "older" than the United States with projected old population percentage at 26 percent by 2050.

"Aging is a global phenomenon. China and India are among the fastest growing in terms of having an older society. That's going to have all kinds of implications," said Marcia Ory, professor and founding director of the Texas A&M Center for Population Health and Aging.

"China is very wise to encourage people to have more children because that will help the dependent ratio," Ory said.

China ended its decades-old one-child policy at the end of 2015, allowing all married couples to have two children.

However, new births in China fell to 15.23 million last year, compared to 17.86 million in 2016 and 17.23 million in 2017.

Ory said countries are worried about the dependency ratio — the number of people working versus the number of people who are old.

"The US Social Security system was founded in 1930s, when there were essentially 17 to 18 workers for each retiree. Today it's down to 3 to 4 workers for every retiree. That is unsustainable based on our current economy and system," Taylor said.

"Now people live to 80, 90 and even 100. The potential care givers are going to be 70 or even 80. What we have is an incredible challenge," said Ory.