Abnormal gait kinematics are common in youth with cerebral palsy (CP), but prior studies have not analyzed their longitudinal change throughout childhood. This study examines how age and orthopaedic surgery influence gait kinematics throughout childhood in those with ambulatory CP. In this institutional review board-approved prospective cohort study, children with spastic CP (GMFCS I-III) were recruited at age 17-40 months. Instrumented gait analysis was performed at 3-year intervals from age 4 to 21 years, collecting longitudinal kinematic data in bare feet at a self-selected speed. The change in Gait Profile Score (ΔGPS) between each pair of gait analyses (intervals) was analyzed by age distribution (<10, 10-15, ≥15 years) and by presence/absence of orthopaedic surgery. The study included 31 children (GMFCS: I [13], II [14], III [4]). A baseline instrumented gait analysis was performed at age 5.8 ± 1.6 years with subsequent analysis at 2.5 ± 1.3-year intervals. Examining ΔGPS from baseline to final outcome, 87% of limbs were improved/unchanged; 298 intervals of ΔGPS were analyzed and classified as nonsurgical or surgical. Analysis revealed greater GPS improvement in intervals with surgery versus intervals without (p = 0.0004). Surgical intervals had significantly greater GPS improvement in the <10- vs. >15-year-old groups, p = 0.0063. Improvement in gait kinematics in children with CP is significantly influenced by age and timing of orthopaedic surgical intervention for gait correction, and was most pronounced for children <10 years old. Although surgery was associated with improved outcomes in all age groups, these improvements were significantly less for children >10 years old. These results reinforce the importance of considering the timing of orthopaedic surgery.