It says some older patients miss out on vital treatment because of their age.
Age discrimination in the NHS has been made illegal but the report says it may still happen when budgets are tight.
Doctors should look at the overall health of a patient instead of using cut-off ages for procedures, it said.
The report, Access All Ages, points out that there are valid reasons why an older patient might not be considered for surgery - because they have other health problems that increase the risk of operations, or that they themselves prefer not to go under the knife.
However, the report said: "While there may be legitimate clinical reasons why an older person may not benefit from surgery, it remains the case that some patients may be missing out.
"Decisions may not always be made on the basis of a comprehensive and objective assessment, but on a series of assumptions about fitness in older age."
It said some doctors may have "outdated perceptions" and a "lack of awareness" about older patients and their ability to cope with surgery.
It highlighted rates of breast cancer, which are at their highest in women above the age of 85. However, the highest surgery rate was in women two decades younger.
Michelle Mitchell, from Age UK, said: "When it comes to people's health, their date of birth actually tells you very little.
"A healthy living 80-year-old could literally run rings round someone many years younger who does not share the same good health.
"Yet in the past too many medical decisions, we believe, have been made on age alone with informal 'cut-offs' imposed so that people over a certain age were denied treatment."
Age discrimination by NHS hospitals was outlawed at the beginning of October in a decision that applied across England, Wales and Scotland. Patients will be able to sue if they are denied care solely because of their age.
Health Minister Dan Poulter said there was "absolutely no place" for assumptions about age in the health service and that patients should receive care "that meets their healthcare needs, irrespective of their age".
The NHS Confederation's chief executive Mike Farrar said the report was "worrying".
He added: "We know that prejudicial attitudes against older people still pervade through society, but the NHS and its staff should close the door to such unacceptable behaviour."
The president of the Royal College of Surgeons, Prof Norman Williams, said: "It is alarming to think that the treatment a patient receives may be influenced by their age.
"There are multiple factors that affect treatment decisions and often valid explanations as to why older people either opt out of surgery - or are recommended non-surgical treatment alternatives.
"The key is that it is a decision based on the patient rather than how old they are that matters."
Assumptions about fitness in older people should not be used to decide whether patients have surgery, according to a report by the Royal College of Surgeons and Age UK.