Inferences are assumed as processes which one draws some conclusions from the premises. Category-based inferences consist of deductive inferences which the category levels in the conclusion are lower than those in the premise and inductive inferences which the category levels are the same or higher than those in the premise. The subjects were given the new informations with the verb `do' (function information) or `have' (part information) in the premise statements, and then required to judge whether the informations were true for the conclusion statements. Deductive inferences were much easier than inductive ones, which were explained by assuming that the subjects drew deductive inferences by generalizing the extensional knowledge about categories in the premise and inductive inferences by the similarity between the premise and conclusion category terms. The age difference was found mainly for the deductive inferences. The inferences in the living-thing category were prompted by the function information, whereas those in the food category were prompted by the part information. Thus, the subjects were sensitive to the category domains within the same natural kinds and the information types. The findings suggested that existing knowledge about the' dynamic category such as living thing was activated by the function information, whereas exixting knowledge about the static category such as food was activated by the prat information.