Rich people live healthy, disability-free lives an average of nine years longer than less wealthy people, according to a major study that lays bare the troubling economic inequalities behind lifespans in the US and UK.

Researchers analyzed data on more than 25,000 adults over 50, looking for factors that could predict how long they lived before they started suffering from age-related disabilities, like being unable to get out of bed or cook for themselves.

The biggest socioeconomic factor in predicting when those problems began was wealth, the team discovered, with richer people enjoying almost an extra decade before experiencing difficulties.

From the age of 50, the wealthiest men analyzed could expect another 31 healthy years of life -- compared with the least well-off, who could only expect another 22 to 23 healthy years.

For women, the wealthiest were projected to enjoy 33 more years of good health, compared with 24 for the poorest.

"While life expectancy is a useful indicator of health, the quality of life as we get older is also crucial," lead author Paola Zaninotto, a public health specialist at University College London, said in a statement. "By measuring healthy life expectancy we can get an estimate of the number of years of life spent in favorable states of health or without disability."

The study -- the work of researchers from a team of universities in Europe and the US -- is far from the first to pinpoint the importance wealth plays in how long people live, though most have focused on life expectancy rather than quality of life.

Research in 2016 found that men in the top financial 1% in the US can expect to live until the age of 87.3, nearly 15 years longer than those in the bottom 1%. The gap for women was 10 years.

And in the UK, a study in 2018 found that poor people die around a decade earlier than those who are better off.

Data for the new research was pulled from two aging studies: one from the UK, which had 10,754 respondents, and another from the US, with 14,803 respondents. There was no significant difference between the two countries in relation to the study's key findings.

"Inequalities in healthy life expectancy exist in both countries and are of similar magnitude," the authors wrote in their conclusion. "In both countries efforts in reducing health inequalities should target people from disadvantaged socioeconomic groups."

In general, the global life expectancy at birth in 2016 -- the latest year for which data is available -- was 72 years, according to the World Health Organization. The global average life expectancy rose by 5.5 years between 2000 and 2016, the fastest increase since the 1960s, WHO said.