A patient's fitness rather than their age should be the key factor in deciding whether they have an operation, according to research.
The study revealed unfit patients cost the NHS an extra £6,000 on average after an operation, due to longer recovery times.
Medics have traditionally seen age as a main risk factor for surgical survival and complications, with older people perceived to come out worse, but researchers from Newcastle University and Newcastle Hospitals NHS Foundation Trust have found that being physically fit is a more important factor in getting a good outcome and a quicker recovery.
Fit older people had a lower risk of dying, recovered better after surgery and had a shorter stay in hospital than unfit younger people, they found.
Professor Mike Trenell, an National Institute for Health Research senior fellow at Newcastle University who led the study, said: "To ensure the best possible outcome after surgery, we have found that it doesn't matter how old you are - it matters how fit you are."
The three-year study, published in the Annals of Surgery, involved a group of 389 adults who underwent elective liver surgery at Newcastle's Freeman Hospital. Their fitness levels were assessed on bike tests before their operation and then their outcomes were monitored.
For patients under 75 and fit, the mortality rate after surgery was less than 1%, while for those over 75 and fit it was 4%.
For patients who were under 75 and unfit, mortality increased to 11% and for those over 75 and unfit it increased to 21%, more than five times the rate of their fit counterparts.
Regardless of age, people who were physically unfit also spent on average 11 days longer in hospital than the fitter ones.
Older people who were described as fit spent the same number of days in hospital as fit younger people.
Prof Trenell said: "This data reinforces how important it is to be physically fit before surgery, no matter how old you are.
We're not talking about being an athlete but fit enough to ride a bike.
"In fact, being older does not necessarily mean that you shouldn't have surgery. But, if a patient is older and has a low level of physical fitness, the care team can now have an informed conversation with them about whether surgery is the best option for them."