Chronic exercise training is associated with an "athlete's artery" phenotype in young adults and an attenuated age-related decline in endothelium-dependent arterial function. Adolescence is associated with an influx of sex-specific hormones that may exert divergent effects on endothelial function, but whether training adaptations interact with biological maturation to produce a "youth athlete's artery" has not been explored. We investigated the influence of exercise-training status on endothelium-dependent arterial function during childhood and adolescence. Brachial artery flow-mediated dilation (FMD) was assessed in <i>n</i> = 102 exercise-trained (males, <i>n</i> = 25; females, <i>n</i> = 29) and untrained (males, <i>n</i> = 23; females, <i>n</i> = 25) youths, characterized as pre (males, <i>n</i> = 25; females, <i>n</i> = 26)- or post (males, <i>n</i> = 23; females, <i>n</i> = 28)-predicted age at peak height velocity (PHV). Baseline brachial artery diameter was larger in post- compared with pre-PHV youths (<i>P</i> ≤ 0.001), males compared with females (<i>P</i> ≤ 0.001), and trained compared with untrained youths (3.26 ± 0.51 vs. 3.11 ± 0.42 mm; <i>P</i> = 0.041). Brachial FMD was similar in pre- and post-PHV youths (<i>P</i> = 0.298), and males and females (<i>P</i> = 0.946). However, exercise-trained youths demonstrated higher FMD when compared with untrained counterparts (5.3 ± 3.3 vs. 3.0 ± 2.6%; <i>P</i> ≤ 0.001). Furthermore, brachial artery diameter (<i>r</i>² = 0.142; <i>P</i> = 0.007 vs. <i>r</i>² = 0.004; <i>P</i> = 0.652) and FMD (<i>r</i>² = 0.138; <i>P</i> = 0.008 vs. <i>r</i>² = 0.003; <i>P</i> = 0.706) were positively associated with cardiorespiratory fitness in post-, but not pre-PHV youths, respectively. Collectively, our data indicate that exercise training is associated with brachial artery remodeling and enhanced endothelial function during youth. However, arterial remodeling and endothelium-dependent function are only associated with elevated cardiorespiratory fitness during later stages of adolescence.<b>NEW & NOTEWORTHY</b> We report preliminary evidence of the "youth athlete's artery," characterized by training-related arterial remodeling and elevated endothelium-dependent arterial function in children and adolescents. However, training-related adaptations in brachial artery diameter and flow-mediated dilation (FMD) were associated with cardiorespiratory fitness in adolescents, but not in children. Our findings indicate that endothelium-dependent arterial function is modifiable with chronic exercise training during childhood, but the association between FMD and elevated cardiorespiratory fitness is only apparent during later stages of adolescence.