(CNN) Life expectancy fell across the majority of high-income countries, signaling a collective and simultaneous decline among affluent nations for the first time in decades, a new study finds.

Among 18 high-income countries -- including Spain, Sweden, Japan, Australia, the UK and the United States -- most countries saw declines in life expectancy between 2014 and 2015, according to the study, published Wednesday in the British Medical Journal.

Australia, Japan, Denmark and Norway were the only countries in the study that showed an increase in life expectancy across all years for both men and women.

The drop in the remaining 14 countries was "notable both for the number of countries and for the magnitude of the declines," the authors wrote.

Outside the United States, declines in overall life expectancy were focused among people 65 and older, with the rise in deaths among this demographic probably attributable to an unusually severe flu season, according to the study, which was co-authored by Jessica Ho of the University of Southern California and Arun Hendi of Princeton University.

It's a trend that highlights some potential issues around health-care provision within these countries, according to Domantas Jasilionis, a research scientist at the Max Planck Institute for Demographic Research in Germany who authored an editorial that accompanied the study

"The fact that modern healthcare systems in the most advanced high-income countries were unable to cope with this unexpected challenge, resulting in the first reductions in longevity for decades, is striking and might signal more profound problems," Jasilionis wrote.

The study also suggested that respiratory disease, cardiovascular disease, Alzheimer's disease and other mental and nervous system disorders contributed in part to driving down life expectancy among those 65 and older.

In countries where environmental threats such as air pollution and poor air quality have become more problematic, deaths from these diseases may become more commonplace, suggests Holly Nelson-Becker, an expert in social gerontology at Brunel University in London, who was not involved in the study.

"The issue of susceptibility to influenza and respiratory problems speaks, in my view, to increasing issues of environmental problems such as smog and other forms of pollution," Nelson-Becker said.

Deaths among young adults in the US

In the United States, the decline in life expectancy was concentrated among a younger cohort of the population: those in their 20s and 30s, according to the study.

It's an alarming trend given the country's already lackluster performance regarding life expectancy among high-income countries, according to the study.

"The USA now has the lowest life expectancy levels among high income developed countries, and Americans fare poorly across a broad set of ages, health conditions, and causes of death compared with their counterparts in these countries," the authors wrote.

The study used trend estimates generated from official vital statistics -- which provide information on population mortality -- from 18 high-income countries: Australia, Austria, Belgium, Canada, Denmark, Finland, France, Germany, Italy, Japan, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, the US and the UK.

Only four countries -- Australia, Japan, Denmark and Norway -- saw a continued rise in life expectancy across the study years. The average life expectancy for women in Australia, Japan, Denmark and Norway in 2016 was 85.46, 87.17, 82.79 and 84.17, respectively. For men, the life expectancy was 81.49, 81.01, 78.95 and 80.61, respectively.

The authors admit they couldn't establish any firm conclusions on cause and effect from the study, so it remains unclear why these four countries were able to retain increases in life expectancy over time.

Continued decline the US and UK

Most countries that demonstrated overall declines between 2014 and 2015 and then rebounded between 2015 and 2016, showing gains in life expectancy to make up for the loss.

But the US and the UK were exceptions to this trend, continuing to show declines in life expectancy in more recent years.

Social inequality, poverty and the declining quality of health-care provision in the US and the UK may be perpetuating this stagnation and decline in life expectancy, suggests Jasilionis. The life expectancy for men and women in the US in 2016 was 76.4 and 81.4 years, while in the UK, those numbers stood at 79.04 and 82.72.

The opioid crisis may be a large contributor to declining rates of life expectancy in the US, but according to a another study by the British Medical Journal published Thursday, mortality rates for organ diseases involving the lungs, heart and other parts of the body have also increased significantly.

Jasilionis adds that unequal access to health care in the US as well as wide variations in its quality may be contributing the country's underperformance in life expectancy.

In the UK, disadvantaged populations who suffer disproportionally from certain social factors, such as poverty, may be driving down life expectancy, Jasilionis said.

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And while many countries did manage to rebound from a dip in life expectancy, the threat of new and emerging diseases, as well as the rising concern over antimicrobial resistance, will pose a particular challenge in the future, Jasilionis said.

"It's too early to say this [drop in life expectancy] won't happen again," he said.

"This will largely depend on if these countries are more prepared in the future to address these threats. There are more unexpected health threats emerging that will require faster responses and more technological development."