How do children become adaptive decision makers in complex environments? Though we know that adult-like abilities are present by age 11 or 12, and that children younger than this often fail to adapt, we know virtually nothing about the mechanisms responsible for this development. In this article, we explore the obstacles that confront young children as they attempt to adapt to complex decision tasks. We focus on the possibility that younger children's failures might be linked to a fairly simple obstacle, such as being insensitive to the cost or effort involved in pursuing alternative strategies for making decisions. This possibility was tested in an experimental setting, with children aged seven to 11 years, in which children's decision-making strategies were monitored as they made choices from increasingly complex information boards in the presence or absence of imposed costs for gathering information from the boards. Our results indicate that age differences in adaptivity can be eliminated with the imposition of search costs, implicating sensitivity to decision-making costs as a major contributor to the development of adaptivity in complex environments.