Peak oxygen uptake (V̇O2) is the criterion measure of young people's aerobic fitness, and blood lactate accumulation (BLA) is a useful indicator of aerobic fitness with reference to the ability to sustain submaximal exercise. In sport and in everyday life it is the pulmonary (p)V̇O2 kinetics of the non-steady state which best assess the integrated responses of the oxygen delivery system and the metabolic demands of the exercising muscle. Data analysis using sophisticated modelling techniques has enhanced understanding of sexual dimorphism and the independent effects of chronological age, body size, and biological maturity on peak V̇O2 and BLA. The extant data on young people's pV̇O2 kinetic responses to step changes in exercise intensity are sparse, but describe intriguing chronological age and sex differences across exercise domains. However, independent effects of biological maturation are yet to be revealed.