Study by Langa, Faul, Weir on dementia prevalence featured in Forbes
"Dementia: The Good, The Bad, The Ugly And A Proposal" - Forbes. 4/27/2019.
A 2017 study using 2012 data found that, based on a study of a representative sample, among individuals aged 65 - 74 years old, 14% had signs of cognitive impairment that was not severe enough to be labeled dementia (CIND, or Cognitive Impairment - No Dementia), and 3.2% met the full definition of dementia. For those 75 - 84 years old, the prevalence rates increase to 22.6% and 9.9% respectively, and for those 85 and over, the prevalence of CIND and full dementia are 29.9% and 29.3%.
Those numbers aren't great. But in each category, they have dropped from the last time this study was conducted, in 2000. For the total study group, all participants 65 and over, the incidence of dementia dropped from 11.6% to 8.8% from 2000 to 2012. With an age-and-sex standardization, the drop is even slightly better, from 11.6% to 8.6%.