A cross-sectional design was used to study the associations among sex, age, maximal aerobic speed (MAS), and running time to exhaustion at 100% of MAS (Tlim). The subjects were 1,448 schoolchildren (685 males and 763 females) ages 6 to 17 years. The MAS of males increased significantly (p < .001) from 6 (9.0 ± 0.8 km · h−1) to 17 years (14.1 ± 1.6 km · h−1). The MAS of females increased significantly (p < .001) from 6 (8.7 ± 0.6 km · h−1) to 12 years (10.7 ± 1.2 km · h−1), then remained relatively constant at approximately 11 km · h−1 between 12 and 17 years. Tlim increased significantly from 6 to 13 years for males and from 6 to 11 years for females, then remained relatively constant around 350 s for both males and females.