Humans may be capable of living to 150, research suggests.
The average Briton dies aged 81, with women generally surviving four years longer than their male counterparts.
While the thought of living another seven decades may sound far-fetched, scientists from the Roswell Park Comprehensive Cancer Center in New York believe humans could still be going strong at 120 to 150 years old.
Writing in the journal Nature Communications, the scientists explain how people become less resilient with age, taking longer to recover from so-called internal stress.
A complete loss of resilience, an inability to recover, may not occur until a person is well into their centenarian years.
This "breakthrough" study could "guide the development of drugs" that slow the ageing process and "extend healthspan".
"This work, in my opinion, is a conceptual breakthrough because it determines and separates the roles of fundamental factors in human longevity: the ageing – defined as progressive loss of resilience, and age-related – as 'executors of death' following the loss of resilience," said study author Professor Andrei Gudkov.
"It explains why even most effective prevention and treatment of age-related diseases could only improve the average, but not the maximal lifespan, unless true anti-ageing therapies have been developed."
Professor David Sinclair, from Harvard, agreed, adding: "The investigation shows recovery rate is an important signature of ageing that can guide the development of drugs to slow the process and extend healthspan."
Ageing is a complex process that begins at birth.
Unhealthy habits – like smoking, drinking excessively and eating poorly – are known to cut years off a person's life, however, the effect of our so-called biological clock was less clear.
Working alongside the Singapore-based biotech firm Gero, the Roswell scientists analysed the blood tests and physical activity level of a group of volunteers over time.
Results suggest healthy individuals are very resilient against the processes that can cause disease. A drop in this resilience is then linked to the onset of ill health and death.
This resilience was found to naturally decline with age, with a 40-year-old taking around two weeks to recover from internal stress, stretching to six weeks among 80-year-olds.
Based on mathematical models, humans are thought to lose the ability to recover at 120 to 150 years old, even if the individual does not have a disease.
A person's life expectancy may therefore not be improved by treating medical conditions alone, but by targeting the root cause of this declining resilience.
"The research will help to understand the limits of longevity and future anti-aging interventions," said Professor Brian Kennedy, from National University Singapore.
"What's even more important, the study may help to bridge the rising gap between the health- and life-span, which continues to widen in most developing countries."