<p>We study in the laboratory age-related changes in the behavior of children and adolescents (ages 7 to 16) in two repeated coordination games, stag hunt and battle of the sexes. In these games, sharing intention and beliefs helps participants reach the efficient and fair long run outcome (EFO). In stag hunt, it means coordinating on the Pareto superior Nash equilibrium, hence a coordination of actions. In battle of the sexes, the exercise is arguably more complex as it requires taking turns between the two static Nash equilibria, hence a coordination of strategies. We find in both games a significant and remarkably stable increase in the ability to coordinate on the EFO with age. At the same time, the majority of participants in all ages adhere to one of a small number of relatively simple strategies. EFO is more prevalent in stag hunt and in the second supergame. This evidence suggests that children gradually learn how to share intentions and beliefs, an ability that can be exported to new interactions, but that is limited by game complexity. More generally, it suggests that dynamic cooperation is not instinctive or innate but rather reflective and acquired.</p>