People who live to be 100 or more often put their longevity down to choices they've made in life - avoiding alcohol, drinking tea and regular exercise are common explanations.
But with others making it to 100 despite their smoking and drinking, scientists have long suspected it could be something in the genes that decides who lives long and who dies young.
New research in Japan has found such a genetic link. Researchers analysed the immune systems of a small number of super-centenarians - people older than 110 - and found that compared to the average person, they have an unusually lethal defence against infections and cancers.
"Exceptionally long-lived people such as supercentenarians tend to spend their entire lives in good health, implying that their immune system remains active to protect against infections and tumours," the study, published in journal Proceedings of the National Academy of Sciences, reads.
"Their characteristics of delayed onset of age-related diseases and compression of morbidity imply that their immune system remains functional."
The scientists found up to 80 percent of T-cells in supercentenarians' immune systems are able to kill cancer cells or normal cells infected with viruses - but in the control group of 50- to 80-year-olds, the figure was just 10 to 20 percent.
"Our study reveals that supercentenarians have unique characteristics... which may represent an essential adaptation to achieve exceptional longevity by sustaining immune responses to infections and diseases."
Earlier this year when 113-year-old Japanese man Masazo Nonaka died, he put his longevity down to eating sweets and taking hot baths. And before 118-year-old Nabi Tajima - also Japanese - died in 2018, she put her long life down to eating cow's feet and reading the Bible.
But the new finding suggests lifestyle factors might not be the biggest influence on whether one makes it to 100 or not.
Japan has a lot of centenarians - about 62,000 - but of them, only about 150 have made it to 110.
The world's oldest woman on record - Jeanne Calment - lived to be 122, but there have been claims she was a fraud. Another researcher said the real key to long life was poor record-keeping, implying most supercentenarians don't really know their true age.