The number of people living with dementia is levelling off- possibly due to healthy lives.
Improvements in health and levels of education might be protecting people from the disease, the scientists said.
The report, in the Lancet medical journal, analysed twinned dementia studies that were conducted in the same way, but decades apart.
Data from five studies from the Netherlands, UK, Spain and Sweden showed that the proportion of people with the condition had stabilised over the periods covered by the studies – which ranged from nearly 20 years to almost 30. But in the UK and among Spanish men, it had fallen.
In the UK, the data from 1991 suggested that 8% of over-65s would have dementia in 2011, yet the team in Cambridge said the figure was in fact 6%.
It means there are around 670,000 over-65s with the condition rather than the 810,000 figure regularly cited.
An ageing population should have led to more people living with dementia. However, lead researcher Prof Carol Brayne said the expected rise “had not occurred”.
“Effectively it has stabilised rather than gone up. The age-specific prevalence has gone down so even though the population has got older, the number of patients with dementia has stayed the same.”
She said the findings suggested there may be a “preventable component within individuals, and across whole populations”.
“It may be possible that we can defer dementia rather than prevent it entirely – it’s very unlikely that we can prevent it entirely.”
The exact reason why rates have fallen is uncertain, but improvements to the health of nations are the most likely. Risk factors for dementia include:
However, it said there was a “new and emerging” picture showing that dementia might not be increasing as rapidly as previously thought.
It said the condition was already a huge social and economic problem, with a quarter of hospital beds filled with dementia patients.
Meanwhile, Dr Matthew Norton, from Alzheimer’s Research UK, said the findings were encouraging, but the risk factors for dementia were “not yet fully understood”.
He also warned: “Current trends in risk factors such as obesity and diabetes mean we should not be complacent. But measures to help people adopt healthy lifestyles now could have a real impact on the numbers of people living with dementia in the future.”