https://www.hattiesburgamerican.com/s...ates/12707053/

Pretty sure it is due to the decline of vascular dementia ( not Alzheimer's) as we improve the risk factors that contribute to vascular dementia ( smoking cessation, better cholesterol and blood pressure control etc.) .

An American over age 60 today has a 44percent lower chance of developing dementia than a similar-aged person did roughly 30 years ago, the longest study of these trends in the U.S. concluded.