People over the age of 90 years—the oldest old—are the fastest growing sector of the population. A substantial proportion of these individuals are affected by dementia, with major implications for the individual as well as society. Research on dementia in the oldest old is important for service planning, and the absence of dementia at this exceptional old age may serve as a model of successful ageing. This Review summarizes population-based epidemiological studies of dementia and its underlying neuropathology in nonagenarians and centenarians. The available data, although somewhat limited, show an age-specific and sex-specific profile of dementia status in very late life, resulting from a variety of neuropathologies that often co-occur. Extensive overlap in neuropathology between cognitively normal and cognitively impaired individuals is evident despite challenges to gathering data particular to this population. A complex picture is emerging of multiple pathogenetic mechanisms underlying dementia, and of the potential risk and protective factors for dementia that interact with genetics and lifestyle in normal and exceptional cognitive ageing.