Objectives: From a longitudinal perspective, the direction of the relationship between cognitive functioning and well-being in old age, both conceptually and empirically, is still under debate. Therefore, we aimed to disentangle the different longitudinal relationship patterns proposed and whether those differed between young-old and old-old adults. Methods: We used latent change score modeling based on longitudinal data from 1,040 older adults (M = 74.54 years at Time 1 [T1], median = 73 years) to analyze reciprocal lead-lag relationships over 6 years in executive functioning (trail making test [TMT] completion time) and well-being (life satisfaction), taking into account chronological age, sex, education, leisure activities, and chronic diseases. Results: In young-old adults (<73 years), longer TMT completion time at T1 (i.e., lower executive functioning status) significantly predicted steeper subsequent decline in well-being. This was not the case for old-old adults ([greater than or equal to] 73 years), for whom this relationship was significantly different from that of the young-old (moderation effect). In either group, well-being status at T1 did not predict changes in TMT completion time. Discussion: Lower executive functioning may predict a subsequent decline in well-being in young-old adults only. Wider implications in a context of promotion of healthy aging are discussed.