The aim of the study was to compare vertical jumping performances in boys and girls during growth. The maximum heights attained in a countermovement jump (CMJ) and squat jump (SJ) were measured using an Ergojump Bosco System. Average power output (PO) was recorded, and percentage of fast-twitch (%FT) muscle fiber distribution was estimated during the rebound jump. Differences in the maximum CMJ and SJ (CMJ–SJ) heights were calculated. Regressions between PO and age, lean body mass (LBM), and leg muscle volume (LMV), respectively, were computed for 240 boys and 239 girls (aged 11–16 years). Height, LMV, and body mass values were larger in boys than girls aged 14 years. Both groups had a similar body mass index independently of age. The CMJ, SJ, PO, and %FT were larger in boys than in girls between 12 and 16 years of age. Strong correlations were found between PO and age in the population as a whole, and between PO and LBM, PO and LMV in each group. The CMJ–SJ decreased with increasing age in both groups without significant differences. Conclusion Jumping performance increases during growth, with gender differences manifesting from 14 years onwards due to the much greater increase in leg length and LMV in boys than in girls.