Background/Objectives: This retrospective longitudinal study analyzed height growth velocity (GV) across skeletal maturity indicator (SMI) intervals in Korean adolescents with skeletal Class II malocclusion to identify the timing and duration of peak height velocity (PHV) and to define a PHV window relevant to growth-modification treatment. Methods: Sixty patients (30 males, 30 females) were followed for at least six years with serial hand-wrist radiographs and height records. GV was calculated for each SMI interval, defined as the period between adjacent SMI stages. In females, the timing of menarche by SMI stage was also recorded. Results: GV increased with skeletal maturation and was greatest at the SMI 5–6 interval in both sexes. In males, GV was already high at SMI 4–5 and did not differ significantly from that at SMI 5–6, whereas in females GV remained high at SMI 6–7 and did not differ significantly from that at SMI 5–6. In all female subjects, menarche occurred after PHV and coincided with growth deceleration. GV declined markedly after SMI 7–8, with minimal growth beyond SMI 11. Conclusions: These findings indicate that the PHV window spans SMI 4–6 in males and SMI 5–7 in females, suggesting that growth-modification therapy may achieve optimal outcomes when initiated within these stages. Because this study evaluated height rather than maxillomandibular growth and individual variation persists, SMI stage alone cannot define PHV, and additional assessments of growth are required when determining the timing of orthopedic intervention.