People turning 100 this year have seen a lot. Born in 1925 between the two world wars, they have witnessed the advent of everything from penicillin to Tiк Toк, reports Euronews.

But very few people have crossed that line. Globally, there are about 630,000 centenarians - people aged 100 and older - and the majority of them live in just 10 countries, according to the latest UN estimates.

These 10 countries span Asia, Europe and North America and vary in size, economic power, politics, quality of health care and social structures.

One in five centenarians, or about 123,000 people, live in Japan, which also has one of the highest life expectancies in the world - 88 years for women and 82 years for men.

This is followed by the United States, where about 74 000 centenarians live, China (about 49 000) and India (38 000).

However, when the number of centenarians relative to population is taken into account, the US and China go down the list, while European countries such as France, Greece and Italy rise closer to the top.

Japan remains the outlier with 100 centenarians per 100,000 population, second only to Hong Kong where the figure is 133. 

"Japan tops all the charts," says Solveig Cunningham, a professor who leads research on health, ageing and longevity at the Netherlands Interdisciplinary Demographic Institute.

This is probably due to a combination of diet, exercise and a healthy "living environment," Cunningham said. But the real test would be to track the health of people who immigrated to Japan decades ago and see if they fare as well as native Japanese as they age.

Extreme longevity has long puzzled scientists and fascinated the public, spawning countless books, TV shows, headlines and self-help guides.

One popular explanation is the theory of "blue zones" - geographic regions where people live remarkably long lives thanks to a combination of healthy diets, active lifestyles, strong social connections and genetics.

But researchers also raise questions about whether these "blue zones" are real, or just a matter of faulty data. 

Last year, a study caused a stir when it pointed to two reasons why there are so many centenarians in some countries: administrative errors in issuing birth certificates and fraud in pension and benefit systems.

Cunningham said there are probably some "outliers in the data," but they are probably not the cause of more general trends, especially because the countries with the most centenarians also tend to have longer life expectancies overall.

Even countries with lower life expectancy may have a surprising number of centenarians.

"In places where early life mortality is quite high, the people who survive are extremely strong," Cunningham says.

There doesn't seem to be a "magic potion," she added. 

So far, there are still more questions than answers when it comes to exceptional longevity, but demographers and health experts say the key to aging is simple: regular exercise, healthy food, access to medical care, getting enough sleep, avoiding alcohol and smoking, and managing stress. | BGNES