The splitting of muscle phosphocreatine (PCr) plays an integral role in the regulation of muscle O₂ utilization during a “step” change in metabolic rate. This study tested the hypothesis that the kinetics of muscle PCr would be faster in children compared with adults both at the onset and offset of moderate-intensity exercise, in concert with the previous demonstration of faster phase II pulmonary O₂ uptake kinetics in children. Eighteen peri-pubertal children (8 boys, 10 girls) and 16 adults (8 men, 8 women) completed repeated constant work-rate exercise transitions corresponding to 80% of the P_i/PCr intracellular threshold. The changes in quadriceps [PCr], [P_i], [ADP], and pH were determined every 6 s using ³¹P-magnetic resonance spectroscopy. No significant (P > 0.05) age- or sex-related differences were found in the PCr kinetic time constant at the onset (boys, 21 ± 4 s; girls, 24 ± 5 s; men, 26 ± 9 s; women, 24 ± 7 s) or offset (boys, 26 ± 5 s; girls, 29 ± 7 s; men, 23 ± 9 s; women 29 ± 7 s) of exercise. Likewise, the estimated theoretical maximal rate of oxidative phosphorylation (Q_max) was independent of age and sex (boys, 1.39 ± 0.20 mM/s; girls, 1.32 ± 0.32 mM/s; men, 2.36 ± 1.18 mM/s; women, 1.51 ± 0.53 mM/s). These results are consistent with the notion that the putative phosphate-linked regulation of muscle O₂ utilization is fully mature in peri-pubertal children, which may be attributable to a comparable capacity for mitochondrial oxidative phosphorylation in child and adult muscle.