The purpose of the present study was to determine the lower extremities biomechanical differences between prepubescent and adult males during drop jumps (DJs). Twenty-four untrained males (12 prepubescents, 12 adults) performed DJs from 20 cm height. Kinematics of the lower extremities were captured, in addition with vertical ground reaction forces (vGRFs) and EMG activity of the gastrocnemius medialis (GM), soleus (SOL) and tibialis anterior (TA) muscles. The results showed that men jumped higher, as expected, but their knees were more flexed prior to landing, and their preactivation level was higher and longer in duration compared to prepubescent boys. During landing, men had shorter contact times, lower vGRF normalized to body mass, and less maximal knee joint flexion. Regarding EMG activity men presented higher stretch reflex and higher EMG activity during the braking phase but the level of coactivation (TA to GM + SOL ratio) was lower. It is seems that pre-landing and landing patterns during a complex task such as DJ are affected by physical development. There are indications that men had higher performance in a DJ than prepubescent boys because they activated more effectively their muscles during the preactivation and braking phase. The above-mentioned data support the hypothesis that prepubescent boys might be inferior in optimal regulation of their muscle-tendon unit stiffness.