Optimal timing for growth tonics is the high-velocity growth period: ① boys age 10–13 (entering puberty), ② girls age 8–11 (before/at menstruation onset). The earlier the better, but ages 6–8 (steady growth phase) and ages 13–15 (late puberty growth phase) also have effects. Past growth plate closure (around 16 for boys, 15 for girls), the effect drops significantly. Determine timing by combining growth plate testing with growth chart analysis.