Intraindividual variability (IIV) in cognitive speed, or moment-to-moment changes in ability, is a developmental phenomenon indicative of neurological integrity that increases gradually across adulthood. Past research has shown that IIV negatively covaries with cognitive performance, in which higher IIV at one occasion is associated with poorer cognitive ability at the same occasion. However, this association has been demonstrated only in older adulthood. Further, all past examinations of IIV change with cognitive change did not remove the average or between-person effect from within-person change in IIV. Using the PATH Through Life Study, we evaluated whether there were differences across 3 age cohorts (20-24, 40-44, and 60-64 years at baseline) in the relationship between 8-year change in IIV and change in cognitive ability (N = 7,485). Change in IIV was partitioned into between-person and within-person components, and multilevel models covarying for education, sex, diabetes, hypertension, and anxiety and depressive symptoms were conducted. IIV was negatively related to baseline cognitive performance at the between-person level. Notably, this relation was apparent and, in fact, strongest for those in young adulthood. Level of IIV was also negatively associated with cognitive change, but primarily for the youngest cohort. In contrast to previous research, there was minimal evidence of significant covariation in which within-person changes in IIV were associated with changes in cognitive performance, regardless of age group. Overall, IIV is a stable characteristic negatively associated with cognition in adulthood, but this link may primarily exist at the between-person level.