Three age groups of children and adolescents (9–10, 13–14, and 17–18 years old) were asked to indicate sources of information which serve as their epistemic authority in nine knowledge areas and to attribute reasons for their choices. In general, the results showed that children and adolescents differentiate among sources and select their epistemic authorities according to knowledge areas. It was found that while the perception of parents as epistemic authorities decreases with age, the perception of self increases considerably and becomes an important epistemic authority. In spite of the decrease in the perception of parents as epistemic authorities, they continue to be a significant source of knowledge. In addition, the perception of friends as epistemic authority increases relatively in social knowledge areas. With regard to causal explanation, it was found that children and adolescents use various reasons to explain their selection of epistemic authority. They differentiated among their reasons on the basis of the selected source. Knowledge-denotating source expertise was used as the most important reason. The selection of oneself was mainly attributed to familiarity, and helpfulness was used to explain the selection of mother, friend and father as epistemic authorities. Friend was also selected because of similarity. Age differences with regard to use of reasons reflected changes in source selection. This line of research sheds light on the interpersonal nature of knowledge acquisition.