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It is unclear whether the longitudinal relation between activity participation and cognitive ability is due to preserved differentiation (active individuals have higher initial levels of cognitive ability), or differential preservation (active individuals show less negative change across time). This distinction has never been evaluated after dividing time-varying activity into its two sources of variation: between-person and within-person variability. Further, few studies have investigated how the association between activity participation and cognitive ability may differ from early to older adulthood. Using the PATH Through Life Project, we evaluated whether between-and within-person variation in activity participation was associated with cognitive ability and change within cohorts aged 20–24 years, 40–44 years, and 60–64 years at baseline (n= 7,152) assessed on three occasions over an 8-year interval. Multilevel models indicated …