The aim of this research was to study the planning of the execution of the Tower of Hanoï task (TOH) through gesture and speech. The effects of age and task complexity on gestures-speech mismatches were analyzed in 144 participants (48 children from 8 to 10 years old, 48 adolescents from 12 to 14 years old, and 48 adults from 18 to 20 years old) during their early explanations of the solution to the problem of the TOH. Results suggested effects from task complexity but not from age. Gesture-speech mismatches could be a possible way to analyze early explanations of the tasks, and the level of difficulty could be considered as a developmental indicator. The question of the relationship between gestures and speech during the planning of complex problems is in fact at the center of a passionate debate on the close relationship between thought and language. It is also at the heart of research on multimodal communication and thinking, according to which human cognition is based on verbal and nonverbal aspects of communicative behavior.