Recent studies stressed the importance of objects comparison for category learning. Recently we compared adults' capacity to learn new categorization principle in a condition in which few exemplars were identified as belong to the same category, versus a condition in which exemplars were identified as belong to different categories. We found that almost all participants learned the categorization rule quite efficiently in the Same-Class Exemplars condition. In contrast, when not provided with explicit directions, many adults failed to learn the categorization rule in the Different-Class Exemplars condition. This was the case even when insuring that the objective information provided in the Different-Class Exemplars condition was sufficient for inferring the rule, and was objectively equally informative as the information provided in the Same-Class Exemplars condition. When we compared adult performance to that of children we found that when presented only with Same-Class Exemplars, young children (6-10 YO) learn the novel categories just as well as older children and adults. However, when presented only with Different-Class Exemplars, unlike older children and adults, young children failed to learn the novel categories. These findings may explain known phenomena in cognitive development, such as the common difficulty of young children in learning subordinate-level categories, and the late emergence of expertise. We suggest that such difficulties results from a cognitive, not a perceptual, immaturity.