Does dementia incidence keep rising after age 90?

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The oldest old are the fastest-growing segment of the US population, and accurate estimates of dementia incidence in this group are important for healthcare planning. Dementia incidence doubles every five years from ages 65 to 90, but it was unknown whether this exponential increase continues beyond 90. The researchers aimed to estimate age- and sex-specific incidence rates of all-cause dementia in people aged 90 and older, including centenarians.

Participants came from The 90+ Study, a population-based longitudinal study of aging and dementia. The researchers followed 330 participants who were aged 90 or older and did not have dementia at baseline, between January 2003 and December 2007. They estimated age- and sex-specific incidence rates of all-cause dementia using person-years analysis.

The overall all-cause dementia incidence rate was 18.2% per year (95% confidence interval [CI], 15.3–21.5) and was similar for men and women (risk ratio, 0.94; 95% CI, 0.65–1.37). Rates increased exponentially with age: 12.7% per year at ages 90–94, 21.2% per year at ages 95–99, and 40.7% per year at age 100 and older. The doubling time based on Poisson regression was 5.5 years.

The researchers concluded that all-cause dementia incidence is very high among people aged 90 and older and continues to increase exponentially with age in both men and women. They said projections of the number of people with dementia should incorporate the continuing increase in incidence after age 90. They also said the results foretell a growing public health burden of dementia as the population ages.