Developmental differences in children's conditional discrimination learning, equivalence-class formation, and equivalence-class disruption were investigated in two experiments. In Experiment 1, children between 2 and 9 years of age demonstrated age-related differences across a series of preliminary training steps, such that time to acquisition was more variable for younger than for older children on an initial identity matching and category matching task. However, upon completion of the preliminary training, there were no age-related differences in time to acquisition of the two arbitrary conditional discriminations that would serve as the basis for equivalence-class formation, nor were there differences in time to demonstrate stable equivalence classes (Experiment 2). Also in Experiment 2, children between 2 and 14 years of age were exposed to a potential challenge to the demonstrated equivalence classes; the reinforcement contingency for the AC conditional discrimination was reversed (i.e., given A1, A2 or A3, reinforcers were produced by selecting C2, C3, or C1 respectively). While there was little change in performance on reflexivity or BA symmetry tests following the challenge, age-related differences were obtained for CA symmetry and combined tests for equivalence. The older children were more likely to demonstrate an orderly change in equivalence-class membership consistent with the reversal training, while the younger children showed either little change or substantial disruption in their equivalence patterns. These data are considered in relation to more traditional investigations of children's category formation, as well as their implications for the study of equivalence-class formation and flexibility.