The present study investigated age-related differences in strategy use in the context of a numerosity judgment task. Eighty-two participants (37 adults, 20 sixth graders and 25 third graders) had to provide numerosity judgments of grids containing from 1 to 49 coloured squares. The choice/no-choice method was used to obtain unbiased estimates of the performance characteristics of strategies. Results showed that: (a) at all ages, the most common strategy was to add the number of coloured squares, (b) strategy distribution and efficiency were influenced by participants? age as well as problem features, and (c) age-related changes in numerosity judgment performance included changes in strategy distribution and efficiency, as well as in strategy adaptiveness (or the ability to choose the most efficient strategy on each problem). Implications of these findings for understanding age-related differences in strategic aspects of numerosity judgment, in particular, and in cognitive development in general are discussed.