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Background: Several risk factors for dementia, Alzheimer's disease, and cognitive impairment have been established; however, knowledge about risk factors in the very old population (>= 85 years) is limited. This study describes the association of several baseline factors with dementia in participants aged >= 85 years, and investigates factors associated with a higher risk of incident dementia over five years.

Methods: The participants in this population-based cohort study were aged 85, 90, and >= 95 years at baseline (2000-2002). Data were collected during home visits for interviews and testing, from a review of medical records, and/or interviewing the caregiver or next of kin. After five years 212 participants could be followed up concerning incident dementia. Multivariate logistic regression was used.

Results: At baseline, 100/353 (28%) of participants had a dementia diagnosis. Over five years, 71/212 (33.5%) participants developed dementia. Few participants with dementia at baseline remained alive after five years (12%). Depression at the baseline and follow-up time were associated with a higher risk of dementia, odds ratio (OR) (95% CI, p-value) 2.91 (1.37-6.16, 0.005) and 1.61 (1.26-2.05, <0.001) respectively. More social contact and a higher Mini-Mental State Examination score at baseline were associated with lower risk of incident dementia, OR (95% CI, p-value) 0.87 (0.78-0.97, 0.009) and 0.83 (0.74-0.93, 0.001) respectively.

Conclusions: Prevalence and incidence of dementia are high in very old people and dementia appears to be a fatal disorder. Depression is associated with higher risk of incident dementia over five years whereas more frequent social contacts and a higher MMSE score are associated with lower risk.