A recently published, observational study  led by Dr. Kenneth Langa shows that the prevalence of dementia in the US has declined significantly from 2000 to 2012.
Making use of the Health and Retirement Study (HRS), a source of nationally representative, longitudinal surveyed data, they compared the dementia rates of the 2000 (n = 10 546) and the 2012 (n = 10 511) waves. Their findings show that dementia prevalence declined from 11.6% in 2000 to 8.8% in 2012.
Another study published in 2016  of participants in the Framingham Heart Study showed that the incidence of dementia in the last three decades had been declining.
Although an increase in total years of education was associated with a lower risk of developing dementia in the first study, many other social and medical factors associated with the onset of the disease are still uncertain.
At this point, we can only speculate about the role that physical activity, leisure time and intellectual stimulation play in these results. What I find very interesting is that this decrease in dementia rates have occurred while the prevalence of hypertension and obesity increases. I would like to see more research done about whether these conditions are protective factors in themselves or the medication prescribed for these conditions -cholesterol-lowering drugs (statins) and antihypertensive drugs- are lowering the rates of dementia in the aging population as some studies have started to suggest.  
 A Comparison of the Prevalence of Dementia in the United States in 2000 and 2012
Kenneth M. Langa, MD, PhD; Eric B. Larson, MD; Eileen M. Crimmins, PhD; et al
 Incidence of Dementia over Three Decades in the Framingham Heart Study
Claudia L. Satizabal, Ph.D., Alexa S. Beiser, Ph.D., Vincent Chouraki, M.D., Ph.D., Geneviève Chêne, M.D., Ph.D., Carole Dufouil, Ph.D., and Sudha Seshadri, M.D.
(This post was originally published on http://www.ourplasticbrains.com on January 7th, 2017)