Is there a limit to how old humans could one day become?
French researchers trying to answer the age-old question said they found evidence in two categories of people who lived long lives of a "biological barrier" to living forever.
The team used the recorded deaths of 1,205 "supercentenarians" who died between 1899 and 2013 aged 110 or older, and of 19,012 athletes who had competed in the Olympics from 1896 to 2012.
The athletes are considered among the longest-living people.
For the supercentenarians, the researchers found longevity had steadily increased until 1997, when the rate levelled out. A similar ceiling was observed among the athlete group.
These trends "provide no signs of a recent increased longevity pattern among the longest-lived," which is not good news for people with average life expectancies, the authors wrote in the Journal of Gerontology.
Had there been a non-stop upward trend, they would have expected to find evidence of more people living ever longer, the team said.
Instead, their results support theories of an "invisible barrier" to a non-stop increase in longevity.
The team conceded the number of people they studied was "relatively small" and the observation period restricted, meaning the trend they observed could be merely a temporary anomaly.