The ability to act jointly with another person is a fundamental requirement for participation in social life. The current study examines the development of action planning in a joint action context. In 4 experiments, 3-, 5-, and 7-year-old children as well as a group of adults (n = 196) interacted with another person to operate a novel apparatus. Their task was to hand the experimenter a tool with which she could activate 1 of 2 different effects on the apparatus. The elicitation of each effect required participants to grasp and insert the tool in a particular orientation. We assessed whether participants planned their grasping and reaching action in such a way that it enabled the partner to efficiently handle the tool, that is, anticipating the final end state of the joint activity. We found that 3-year-old children did not adjust their behavior to accommodate the other's action and that they did not increase their performance over multiple trials. Five- and 7-year-old children initially showed a tendency to plan their action in an egocentric manner (i.e., showed a form of egocentrism), but improved their joint action performance over time. Adult participants demonstrated joint action planning from the beginning. Interestingly, 3- and 5-year-old children were able to plan their grasp efficiently when acting alone on the apparatus. Yet, having first-hand experience with the task before acting with a partner did not facilitate performance in the joint action task for younger children. Overall, the study informs current approaches on the psychological basis and ontogenetic origins of joint action in childhood.