(CNN) Average life expectancy will increase globally by 2030, both at birth and at the age of 65, according to a new study.

The two time points help define when lifespans in a population are extending due to improvements in maternal and child health as well as improved adult health.

The average for women at birth will exceed 85 years in many countries, but South Korea is projected to lead the way with a life expectancy of 90.8 years.

In 2015, global average life expectancy at birth was 71.4 years, according to the World Health Organization.

Unhealthier lifestyles among men, including higher rates of smoking and alcohol consumption, have long meant a greater life expectancy for women, say the researchers.

Life expectancy among men born in South Korea in 2030 is therefore predicted to be 84.1 years, according to the study, published Tuesday in the Lancet.

But the study authors believe this gap between the sexes is now shrinking as lifestyles become more similar.

"The biggest result is that ... at least one group is going to break the 90-year barrier," said Majid Ezzati, professor of global environmental health at Imperial College London, referring to the predictions among South Korean women. Ezzati led the study and highlighted that many experts had believed the average would never exceed 90.

"This shows that even if there is a limit to longevity, we are nowhere near it," he said. "We should be planning for more life."

The analysis included data on mortality and longevity patterns from 35 industrialized nations, including both high-income countries and emerging economies.

US continues to lag

In Europe, French women and Swiss women are predicted to have the highest life expectancy, with averages of 88.6 and 84 years respectively.

Among predictions for high-income countries, the lowest life expectancy at birth is likely to be in the US, with an average of 83.3 years for women and 79.5 years for men -- similar to Mexico and Croatia.

"They still have relatively high rates of young and middle-age mortality," Ezzati told CNN, referring to the countries with lower life expectancies. "People have a relatively high risk of dying in their 40s or 50s."

There are many reasons for deaths in this age range, Ezzati said, including greater obesity rates and their associated health risks as well as homicides and road accidents. Lack of universal health care in countries like the US is also thought to play a role, the study says.

The lowest life expectancy projections overall were for women in Macedonia and men in Serbia, according to the study.

"A lot of this is driven by inequality, (which) brings down the national average," Ezzati said.

The research team calculated life expectancy using data on the age at which people die across a population. High childhood mortality rates and greater rates of death among young people, for example due to injuries and violence, will bring an average down.

In contrast, greater equality, more comprehensive health care and healthy diets and lifestyles embedded in a culture can bring this up -- as with South Korea.

"Places like South Korea and some emerging economies, Switzerland and Canada do very well," Ezzati said. "South Korea has had remarkable success."

Ezzati believes the secret to South Korea's success has been the country's investment in childhood nutrition, education and technology as well as low blood pressure, low levels of smoking and good access to health care.

"An equitable way of taking up this knowledge has been the driver," he said.

Overtaking Japan's lead

Meanwhile, countries like Japan, long revered for their longevity, are expected to see just small increases in life expectancy. "The Japanese story has started to come to an end," Ezzati said, adding that while the Japanese culture is known for healthy diets and active lifestyles, "that's beginning to change."

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"They still have low obesity and low blood pressure ... and are still doing well, but some things are catching up," he said, highlighting Western influences such as diet.

"South Korea has caught up with Japan as its standard of living has increased," said Sarah Harper, professor of gerontology at the University of Oxford. "But in many parts of Asia, young people are eating Western diets. ... The (previously) healthy diet may not be sustained as young people reach older ages."

The findings are hoped to help countries learn healthy changes by example and, more important, to help policymakers prepare for their populations' changing demographics.

"What South Korea has done goes completely against some Western countries ... against the prosperity agenda," Ezzati said, adding that we need to think about "social care, planning and pensions in an aging population."

Harper, said the new projections are, "very much in line with what other projections are showing." She agreed that countries in Asia are leading the way but are also seeing change among their younger population.

She further agreed that inequality is likely to be an underlying factor for why some high-income countries are seeing lower life expectancy predictions. "The US spends more of its budget on health care than any other country in the world. Some say this is due to the huge amount of inequality," she said. "The Scandinavian countries do well. ... They're counted as more equal and tend to have higher life expectancy outcomes."