Aspectual and mood morphology are vulnerable domains in adult heritage speakers. This paper investigates the root of such vulnerability within the domain of Turkish evidentiality system by comparing 20 second-generation adult and 20 school-age child Turkish heritage speakers to 20 first-generation immigrants (main input providers for child heritage speakers) residing in the United States. A monolingual group from Turkey including 20 monolingual adults, 20 7-14-year-old and 20 3-6-year-old Turkish-speaking children were also tested. Results of a story-telling and a sentence selection task revealed weak mappings between the indirect evidential marker --mIs and its pragmatic use in story-telling as opposed to intact knowledge of the Turkish evidentiality system in the first-generation immigrant group. The school-age child and adult heritage speakers, on the other hand, were significantly the least accurate groups in both tasks, performing more variably than the 3-6-year-old monolingual children in Turkey. The individual analysis of the child heritage speaker data as well as a comparison of the child-parent dyads further showed that early heritage language experience and use rather than changes in the parental input are likely to be the source of the variability observed in child and eventually in adult Turkish heritage speakers in this domain.