This study assessed the extent of aerobic system activation in young adolescents and men during heavy continuous (HC), short-interval (SI), and long-interval (LI) aerobic exercise protocols, and compared this response between the 2 age groups in the 3 protocols. Ten young adolescents (aged 13.2 ± 0.3 years) and 10 men (aged 21.0 ± 1.6 years) completed a maximal incremental test, an HC exercise protocol (83% of maximal aerobic velocity; MAV), an SI exercise protocol (30 s at 110% MAV with 30 s at 50%), and an LI exercise protocol (3 min at 95% MAV with 3 min at 35%). Oxygen consumption and heart rate were measured continuously, and blood samples were obtained for lactate determination. Men completed more runs and distance in the SI protocol (p < 0.05) than adolescents; however, there were no age differences in the number of LI runs and in the duration of HC protocol. In both age groups, more time was spent above 90% and 95% of maximal oxygen consumption (p < 0.05), and a higher percentage of maximal oxygen consumption was reached in the LI compared with the HC and SI protocols, with no differences between the HC and SI protocols. Although within each protocol the percentage of maximal oxygen consumption achieved and time spent above 90% and 95% of maximal oxygen consumption was not different between age groups, the time spent at 80% maximal oxygen consumption was longer for adolescents than men in the HC protocol, and longer for men than boys in the SI protocol (p < 0.05). In conclusion, all protocols elicited high levels of aerobic activation in both age groups. The LI protocol taxed the aerobic system at 90%-100% of maximal oxygen consumption for a longer time when compared with the HC and SI protocols in young adolescents and in men. However, differences were observed between groups in taxing the aerobic system at 80% maximal oxygen consumption: in young adolescents, the HC protocol allowed longer running time than the LI and SI protocols, while in men there were no differences among protocols.