Purpose: The aim of our study was to investigate age-related differences in childhood and adolescent boys’ performance, including initial reaction time and electromyographic (EMG) latencies of leg muscles during sprint start. Methods: Forty-six Japanese boys, aged 5–19 years, participated in this study. The participants performed 5 consecutive 2-m sprints, starting with the boys’ preferred split-stance starting posture. Ground reaction force data of the front and rear feet and surface EMG data of the rectus femoris, biceps femoris, and gastrocnemius muscles in both legs of all participants were measured using a wireless EMG system. Results: As chronological age increased, the initial reaction time was largely shortened until the breakpoint age of 8.6 years and was moderately shortened thereafter. The EMG latencies of the posterior thigh and shank muscles of the rear leg were also shortened largely long before reaching the adolescence; in contrast, those of the front leg muscles were largely shortened until adolescence. Conclusions: Reaction behavior with complex whole-body motion may develop long before the onset of adolescence. In particular, adjusting the rear leg motion may contribute to the rapid development of whole-body reaction maturation.