Background: Epidemiological studies commonly include too few of the oldest old to provide accurate prevalence rates of dementia in older age groups. Estimates of the number of those affected, necessary for healthcare planning, are thus flawed. The objective is to estimate the prevalence of dementia and levels of dementia severity in a very large population of oldest old and to investigate the relation between age and dementia prevalence in the extreme ages.

Methods: The Monzino 80-plus is a population-based study among residents 80 years or older in Varese province, Italy. Dementia cases were identified using a one-phase design. The survey was conducted in the participant's place of residence, whether home or institution. Both participants and informants were interviewed. Information was available for 2504 of the 2813 residents (89%).

Results: In all, 894 individuals (714 women and 180 men) met the Diagnostic and Statistical Manual of Mental Disorders (fourth edition) criteria for dementia, for a standardized prevalence of 25.3% (95% confidence interval [CI]: 23.4, 27.2%), 28.5% (95% CI: 26.2, 30.9) in women and 18.6% (95% CI: 15.2, 21.9) in men. Age-specific prevalence estimates of dementia increased with age from 15.7% at age 80 to 84 years to 65.9% at age 100 years and higher. For women, prevalence continued to rise after age 100 years, from 64.8% at age 100 to 101 years to 76.1% at age 102 to 107 years. After age 85 years prevalence rates tended to rise linearly, on average 2.6% per year in women and 1.8% in men. About 80% of the cases were moderate or severe. The frequency of mild dementia decreased and that of severe dementia increased with age.

Conclusion: One-quarter of 80-plus year olds are affected by dementia, mostly moderate or severe. Prevalence rates of dementia do not level off, but continue to rise gradually even in the extreme ages.