In 2012, Taylor Swift sang about feeling “22.” In 1966, the Beatles sang “When I’m Sixty-Four.”

While no one’s necessarily singing about this, a study published in The Lancet Public Health took a look at what age people around the world start feeling 65 and found an astonishing 30-year gap. Most Americans said they experienced the health problems of an “average 65-year-old” at age 68.5.

Researchers at the Institute for Health Metrics and Evaluation at the University of Washington in Seattle conducted the study to determine how well or poorly people age throughout the world. Age-related health problems have a significant impact on many facets of society, including the workforce — by forcing early retirement, for example — and the health care system.

The UW researchers used data from the Global Burden of Disease study to analyze 92 conditions commonly associated with aging, including five communicable diseases, 81 non-communicable diseases, and six injuries. They also evaluated data on "age-related disease burden" in 195 countries from 1990 to 2017.

Using a group of “average” 65-year-olds as a reference, they estimated the ages at which people in each country experienced the same level of disease burden. Japan ranked highest in aging well, with 76-year-olds there experiencing the same age-related disease burden as 46-year-olds in Papua New Guinea, which ranked last.

From 1990 to 2017, 108 countries experienced an increase in health problems linked to aging, and 87 countries experienced a slower onset of age-related health problems. Worldwide, the most common health problems were ischemic heart disease, brain hemorrhage, and chronic obstructive pulmonary disease (COPD).