Some researchers and many in the lay public believe the ageist myth that the older you get the sicker you get. If this were true, it would follow that most if not all centenarians should have Alzheimer's disease. Numerous centenarian studies disprove this assumption given that a small percentage ( approximately 15-25%) of centenarians are functionally cognitively intact. Among those who are not cognitively intact at 100, approximately 90% delayed the onset of clinically evident impairment at least until an average age of 92 years. Neuropsychological and neuropathological correlations thus far suggest that there are centenarians who demonstrate no evidence of neurodegenerative disease. There also appear to be centenarians who, despite the substantial presence of neuropathological markers of Alzheimer's disease, do not meet clinical criteria for having dementia, thus suggesting the existence of cognitive reserve. Centenarians are therefore of scientific interest as a human model of relative resistance to dementia.