Evidentiality in language marks how information contained in a sentence was acquired. For instance, Turkish has two past-tense morphemes that mark whether access to information was direct (typically, perception) or indirect (hearsay/inference). Full acquisition of evidential systems appears to be a late achievement cross-linguistically. Currently, there are two distinct hypotheses about why this is so. According to the first hypothesis, the acquisition of evidentiality is delayed by conceptual factors related to source monitoring (the process of identifying and evaluating information sources). According to a different hypothesis, a substantial part of the learning difficulty comes from mapping evidential markers onto the underlying source concepts (even if these concepts are already available to the child), most likely because source concepts do not correspond to observable referents in the world. Here we tested these two hypotheses in a series of experiments comparing the acquisition of evidential morphology (Experiments 1-3) and the development of source monitoring (Experiments 4-6) in the same group of Turkish-speaking children. We found that the semantics and pragmatics of evidential morphology in Turkish are not acquired until age 6 or 7. A comparison between linguistic evidentiality and source monitoring experiments revealed that conceptual understanding of information access develops before the corresponding concepts are linked to evidential morphemes in Turkish, thereby demonstrating that mapping difficulties underlie the late acquisition of evidentiality in Turkish. Nevertheless, our data also suggest that conceptual limitations play an important role in the acquisition of evidentiality, since in both language and source monitoring direct evidence seems to be privileged compared to indirect evidence. This work has implications for the acquisition of mental-state language and the relation between children's linguistic and conceptual development.