Despite expectations that an ageing population would lead to more cases of the illness being diagnosed, scientists at Cambridge University said they had gathered “compelling evidence” of a drop.
Studies funded by the Medical Research Council interviewed more than 7,000 people in Cambridge, Newcastle and Nottingham between 1989 and 1994 and another similar-sized group between 2008 and 2011.
The earlier studies had estimated a total of 664,000 people suffering from dementia across the UK in 1991, rising to 884,000 in 2011.
However, the later survey suggested the actual number of people with dementia in 2011 was 670,000 – a reduction of 24 per cent on what had been predicted.
The researchers said better education about dementia and improved levels of vascular health seemed to have offset factors which would normally increase the prevalence of the illness, such as increasing numbers of people with diabetes or those surviving stroke or heart disease.
Professor Carol Brayne, of the Cambridge Institute of Public Health (CIPH), who led the study, said: “This study provides compelling evidence of a reduction in the prevalence of dementia in the older population over two decades.
“Whether or not these gains for the current older population will be borne out in future generations would seem to depend on whether further improvements in primary prevention and effective healthcare for conditions which increase dementia risk can be achieved.”
Prevalence of dementia remains considerably higher in females, with 7.7 per cent of women over 65 thought to have the illness, compared with 4.9 per cent of men.
The research, published in medical journal the Lancet, suggests that while overall prevalence of dementia has fallen, the number of people living with the illness in care homes has increased from about 56 per cent 20 years ago to 70 per cent today.
However, the researchers noted that this was likely to be a result of people moving into care homes later in life, compared with the past.
Kirsty Yanik, awareness manager for Alzheimer Scotland, welcomed the findings but said more research was needed.
She said: “The results of this dementia prevalence study are extremely interesting – it could indicate that the pre-Second World War generation has benefited from better healthcare and adopted healthier lifestyles, thus reducing dementia risk.
“However, it is just one study and we would need to see further evidence of this in future research. Alzheimer Scotland encourages everyone to stay physically, socially and mentally active, as well as maintaining a healthy diet and drinking only in moderation, as there is evidence that these measures can help to reduce your risk of developing dementia.”