Being richer and healthier provides no protection against developing dementia, a Medical Research Council study says.
The study looked at 13,000 over 65s in England and Wales
It involved 13,000 over 65s in five areas of England and Wales, the Public Library of Science Medicine reports.
Researchers found people in affluent areas, where there is better health and longer life expectancy, were no less at risk of developing the brain condition.
They also found the risk of dementia did not tail off as a person got well into their 80s as other studies have.
Campaigners said this was likely to mean the number of people with dementia increased with the ageing population.
The over 80s population is predicted to double to five million by 2031, according to the Office of National Statistics.
The MRC study interviewed people from five locations with different risks of cardiovascular disease - rural Cambridgeshire, Oxford (low risk) Anglesey, Newcastle (high) and Nottingham (medium) - and found the risk of developing dementia was broadly the same.
The report said in individuals aged 75 to 79 one in 70 people developed dementia each year.
While in the 85-plus age group, this increased to one in 15.
The study estimated that about 163,000 new cases of dementia occurred in England and Wales each year.
Lead researcher Dr Fiona Matthews said the results were unexpected.
And she added: "The new research will help those planning services for people with dementia to estimate requirements now and in the future."
Experts in Alzheimer's disease, the most common cause of dementia, agreed the findings were surprising.
Dr Susanne Sorensen, head of research at the Alzheimer's Society, said it showed dementia did not discriminate between "rich and poor".
And Harriet Millward, deputy chief executive of the Alzheimer's Research Trust, said the finding that the risk of dementia increased as a person got into their 80s had important implications.
"It means dementia could become an even greater problem than already thought as the population continues to age," she said.
"It highlights the urgent need for more medical research to find ways to prevent, cure or treat dementia.
"It could also have large implications for health care costs since an Alzheimer's Research Trust study showed that the cost of caring for people with Alzheimer's disease was more than for stroke, heart disease and cancer put together."
Gordon Lishman, director general of Age Concern England, said: "Increased funding must go into providing special equipment and safety gadgets to protect dementia suffers at home, and providing hard-working carers with decent respite services and help with handling their finances."