Differential object marking (DOM) is an area of vulnerability in adult heritage speakers. This study traces such vulnerability to childhood by examining Turkish DOM in child Turkish heritage speakers in the U.S and the parental generation, who are the main input providers. Twenty first-generation immigrants, 20 adult and 20 child (aged 7–14) Turkish heritage speakers, and the monolingual group including 20 Turkish-speaking adults, 20 7–14-year-old and 20 3–6-year-old Turkish-speaking children in Turkey completed a story retelling task and a picture selection task. Results showed that the first-generation immigrants patterned with the monolingual adults. However, the heritage speakers (children and adults) omitted DOM in both tasks, showing more variable performance than the monolingual groups. These findings suggest that instability of DOM in heritage grammars is more likely due to insufficient input in the early years of heritage language development than to changes in parental input or attrition in later years.
