The relative frequency of "dementia of unknown etiology" increases with age and is nearly 50% in nonagenarians.

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Abstract
IN PATHOLOGICAL studies, the relative frequency of dementia that is not caused by Alzheimer disease (AD), dementia with Lewy bodies (DLB), or frontotemporal dementia ranges from 2% to 65%.1-8 One reason for this wide range of values is that pathological definitions for these disorders have changed during the past 20 years to achieve a better mapping between the clinical syndromes of dementia and pathological findings. For example, numerous nondemented elderly subjects met Khachaturian pathological criteria for AD.9 This mismatching led to the more restrictive Reagan Institute–National Institute on Aging (NIA) criteria,10 which yield fewer cases of AD than the earlier Khachaturian criteria. More restrictive criteria for AD also yield a larger group of patients whose pathological features fail to fit the definitions of well-characterized entities.