https://www.nytimes.com/2025/03/22/health/dementia-rates-elderly.html

For 85-89yo prevalence 23% in 1905 cohort, 1915 18%, 1935 11%, 1945 8%.

Reasonable explanations include BP meds, statins, decreased smoking, better air quality. I’d add fewer lifetime head injuries — Boxing was extremely popular from 1920s to 1940s and nobody wore helmets at work or play. I suspect the incidence will stabilize at around 8% though.