Physical activity declines from childhood to adolescence. Affective factors may partially account for this decline. The present study investigated whether within-person changes in children’s enjoyment of physical activity are associated with the age-related decline in physical activity. Children (N = 169, 54% female, 56% Hispanic; 8–12 years old at enrollment) took part in a longitudinal study with six assessment waves across 3 years. At each wave, enjoyment of physical activity was reported, and moderate to vigorous physical activity (MVPA) was measured with an accelerometer across seven consecutive days. MVPA and enjoyment of physical activity both declined across waves. Multilevel analyses revealed that within-person changes in enjoyment moderated the effects of age on within-person changes in MVPA. Enjoyment appeared to be a dynamic factor that buffered against the age-related decline in physical activity in youth. These findings call for health promotion interventions that encourage enjoyable physical activities.