AbstractBackgroundRecent 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.MethodsWe 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.ResultsThe 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.ConclusionsThe 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.