PURPOSE OF REVIEW: People over 90 are the fastest growing segment of the population with the highest rates of dementia. This review highlights recent findings that provide insight to our understanding of dementia and cognition at all ages. RECENT FINDINGS: Risk factors for Alzheimer's disease (AD) and dementia differ by age, with some factors, like the development of hypertension, actually becoming protective in the oldest-old. At least half of all dementia in this age group is due to non AD pathologies, including microinfarcts, hippocampal sclerosis and TDP-43. The number of pathologic changes found in the brain is related to both risk and severity of dementia, but many people in this age group appear to be 'resilient' to these pathologies. Resilience to Alzheimer pathology, in part, may be related to absence of other pathologies, and imaging and spinal fluid biomarkers for AD have limited utility in this age group. SUMMARY: Studies of dementia in the oldest-old are important for our understanding and eventual treatment or prevention of dementia at all ages.