The present paper provides a critical overview of how adult neuropsychological models have been applied to the study of the atypical development of numerical cognition. Specifically, the following three assumptions are challenged: 1. Profiles of strength and weaknesses do not change over developmental time. 2. Similar neuronal structures are activated in children and adults, as well as the notion that 3. Similarities in behavioral performance imply equivalence in underlying neurocognitive mechanisms. Data from behavioral and neuroimaging studies with both typically and atypically developing children is reviewed to illustrate the pitfalls of these assumptions. The present review proposes that, instead of resting on adult neuropsychological models, the use of both cross-sectional and longitudinal methods is required to elucidate the age-related changes in brain and behavior that give rise to the breakdown of numeracy and mathematics. Empirical data derived from such studies will generate explanatory models of the development of atypical numerical cognition.