The main question we try to answer in this paper is whether stage-like progression in cognitive development can be explained by transfer of cognitive skill among tasks. We focus on the following question: To what extent does training on one task improve the performance on another task? The tasks are Piaget’s (1959) Balance Scale Task and Number Conservation Task, and a task that we will call the Une-Sentence Task, which is taken from Karmiloff-Smith's (1979) experiment on the acquisition of determiners in French. We re-implemented already existing models within the framework of the PRIMs cognitive architecture (Taatgen, 2013). Each task was subdivided in certain stages related to the complexity of the problem-solving strategies. We show that mastery of a certain stage of a problem becomes easier if a higher stage of another task is mastered first.