Background: Recent studies suggest a decline in the age-specific incidence and prevalence of dementia. However, results are mixed regarding trends among octogenarians. We investigated time trends in the prevalence and incidence of dementia in 3 population-based cohorts of 85–90-year olds. We also examined if there were different time trends for men and women.

Methods: We examined population-based birth cohorts within the Gothenburg H70 Birth Cohort Studies born 1901–02, 1923–24, and 1930, at ages 85 (N = 1481) and 88 (N = 840) years. The first 2 cohorts were also examined at age 90 (N = 450). The incidence was examined in 1 109 individuals free from dementia at baseline using information from the examination at age 88 or register data. All 3 cohorts were examined with identical methods.

Results: The prevalence of dementia decreased from 29.8% in 1986–87 to 21.5% in 2008–10 and 24.5% in 2015–16 among 85-year olds, and from 41.9% in 1989–90 to 28.0% in 2011–12 to 21.7% in 2018–19 among 88-year olds, and from 41.5% in 1991–92 to 37.2% in 2013–14 among 90-year olds. The decline was most accentuated among women. The incidence of dementia per 1 000 risk-years from ages 85 to 89 declined from 48.8 among those born 1901–02 to 37.9 in those born 1923–24 to 22.5 among those born 1930.

Conclusions: The prevalence and incidence of dementia decreased substantially over 3 decades among octogenarians. This might slow down the projected increase in cases of dementia expected by the increasing number of octogenarians during the following decades.