Cooperation among children can appear haphazard, a finding often attributed to deficient social skills and moral reasoning. Here we took a game theoretical approach to understand development of cooperation, using the prisoner’s dilemma to test an alternative source of age-differences in cooperative behavior—how children and adults represent the numerical magnitudes of payoffs for cooperating versus not. We found that as incentives increased solely in numerical magnitude, speed of incentive comparisons decreased and cooperation increased. Further, though children tended to be more cooperative than adults, effect of age on cooperation was moderated by speed of incentive comparison. We conclude that representations of numeric value constrain how economic rewards affect cooperation and that children’s greater cooperativeness may be attributed to a poor sense of numerical value.