Which Age Matters? Comparing Chronological and Biological Age in Adolescent Adaptation to School-Based Physical Activity Interventions (Wrocław PEER-HEART Study)

<b>Background/Objectives:</b> Relative age differences within the same school year may influence adolescents' physiological adaptations to physical activity programs. While biological maturity (maturity offset, MO) is often considered a more relevant indicator than chronological age (CA), empirical evidence from school-based interventions remains limited. This study aimed to compare the predictive value of CA and MO in explaining health-related adaptations to an eight-week high-intensity interval training (HIIT) program delivered during physical education classes. <b>Methods:</b> A total of 256 adolescents (112 boys and 144 girls) participated in HIIT protocols integrated into regular lessons. Health-related outcomes included maximal oxygen uptake (VO₂max), body fat percentage (BFP), systolic blood pressure (SBP), and diastolic blood pressure (DBP), assessed at baseline, post-intervention, and follow-up. Maturity offset (MO) was estimated using Moore's method. Statistical analyses included MANOVA/ANOVA, linear regression, and dominance analysis, all stratified by sex. <b>Results:</b> Chronological age explained more variance in the studied outcomes than MO, particularly for BFP and VO₂max among girls. In boys, a significant CA × MO interaction predicted SBP, indicating interdependence between both age indicators. Dominance analysis confirmed the overall predominance of CA as a predictor across most outcomes. <b>Conclusions:</b> Chronological age proved to be a stronger predictor of adaptation to school-based HIIT than biological maturity, suggesting that calendar age better reflects cumulative behavioral and environmental influences. These findings highlight the need for age-sensitive and personalized approaches when designing physical education interventions to optimize health-related outcomes in adolescents.