There is contrasting evidence on personality stability in advanced age, and limited knowledge on the impact of cognitive impairment and dementia on trait stability. Group- and individual-level longitudinal analyses of the five major dimensions of personality assessed twice over 4 years (N = 9,935) suggest that rank-order stability was progressively lower with advancing age (from rtt = 0.68 for age 50 to 60 years to rtt = 0.58 for age >80 years). Stability was low in the dementia group (rtt = 0.43), and this was not simply due to lower reliability given that internal consistency remained adequate in the dementia group. Among individuals with no cognitive impairment or dementia, there was no association between stability and age (rtt = 0.70 even for age >80 years). These results suggest that the lower personality stability in older adults is not due to age but cognitive impairment and dementia.