We report the results of two studies that investigate the factors contributing to non-native-like ability in child and adult heritage speakers by focusing on oral production of Differential Object Marking (DOM), the overt morphological marking of animate direct objects in Spanish. In study 1, 39 school-age bilingual children (ages 6-17) from the United States and 20 monolingual children from Mexico completed a Story Retelling Task and a Picture Description Task. In study 2, 64 young adult heritage speakers (ages 18-25), 23 adult immigrants to the United States (ages 40-60), and 40 native speakers from Mexico (ages 18-60) completed the same oral tasks. Results showed significant rates of omission of DOM in animate direct objects in all the experimental groups from the United States and ceiling performance in the groups from Mexico (both children and adults). We discuss how the combined effects of reduced input, potential attrition in the first generation of immigrants, incomplete acquisition in the second generation, and transfer from English may account for the persistent patterns of DOM omission with animate and specific direct objects in child and adult Spanish heritage speakers. (Contains 7 tables, 7 figures and 3 footnotes.)