Three quarters of women over the age of 50 do not realise that breast cancer is more common as women age, according to a survey.
Breast x-ray: Many women ignore mammograms
Many are also ignorant about other facts which doctors say could save their lives by allowing them to seek treatment early.
One in five women over 50 do not check their breasts for changes from time to time, and the same percentage fail to attend breast screening.
It is hoped the survey, commissioned by charity Breast Cancer Care, will boost women's knowledge about the disease.
The charity's chief executive Christine Fogg said: "It is crucial that all women - and especially women over 50 - understand that the strongest risk factor for breast cancer is age, and that 80% of breast cancers occur in women over 50.
"We are worried that the lack of knowledge in women may delay detection of breast cancer."
Breast cancer is now the most common cancer for women in the UK - a woman has a one in nine risk of developing it at some point in her life.
Experts say that to speed early detection - but cut false alarms - women should be "aware" of their breasts and any new changes such as lumps, and should make sure they attend breast screening sessions.
But fewer than half of the women in the survey, which was supported by high street chemist chain Boots, said they were confident they would know which changes were worthy of being reported to their GP.
Another 13% mistakenly said they did not think that women were at risk after the age of 70.
Screening age limit
Emma Pennery, from the Royal College of Nursing's Breast Care Forum, said: "It is very disturbing to see that so many older women are still not aware of the basic facts of cancer and breast awareness.
"Women themselves can play such an important role in early detection, which can mean simpler treatments and better survival rates."
Age Concern wants the upper age recommendation on organised breast screening to be raised beyond the current limit of 70 years.
Director General Gordan Lishman said: "This important survey highlights a dangerous misconception which still persists: that breast cancer is more likely to affect younger rather than older women.
"Yet the reality is that 80% of breast cancers occur in women over 50 and the risk of being affected usually increases with age."