A survey of 7796 older adults (65+) living in three geographic areas in England has allowed us to compare dementia rates over time, with an identical survey having been taken between 1989 and 1994. The overall prevalence of dementia fell significantly, from 8.3% to 6.5%.

The finding provides further evidence that a cohort effect exists in dementia prevalence.

http://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2813%2961570-6/fulltext

[3589] Matthews, F. E., Arthur A., Barnes L. E., Bond J., Jagger C., Robinson L., et al.
(2013).  A two-decade comparison of prevalence of dementia in individuals aged 65 years and older from three geographical areas of England: results of the Cognitive Function and Ageing Study I and II.
The Lancet. 382(9902), 1405 - 1412.