A new study finds that people over 90 have the highest rates of dementia and Alzheimer’s rates, which counters previous beliefs that the diseases tapered off in the oldest old.

According to previous research, the rate of dementia begins to level off and even decline among people aged 90 and older. However, this new study found the rate of dementia increases with age, HealthDay reports.

Researchers conducted a population-based study of all people aged 80 or older in eight municipalities of Varese province in Italy. They were assessed at the start of the study, and the survivors were assessed again an average of three years later.

The study found that dementia prevalence (total number of cases) increased with age: 13.5 percent at 80 to 84 years; 30.8 percent at 85 to 89 years; 39.5 percent at 90 to 94 years; and 52.8 percent among those older than 94. New cases of dementia also increased with age: 6 percent at 80 to 84 years; 12.4 percent at 85 to 89 years; 13.1 percent from 90 to 94 years; and 20.7 percent among those over 94.

“Gathering reliable information on such a large number of the ‘oldest old’ makes this one of the largest studies investigating dementia in this age segment of the population,” study author Ugo Lucca, head of the Laboratory of Geriatric Neuropsychiatry at the Mario Negri Institute for Pharmacological Research in Milan, said. “This study’s results confirm that Alzheimer’s and dementia are very common among the oldest people in society. We believe this strengthens the need to shift more of the focus of clinical research to this segment of the elderly population.”