The current study investigates case morphology development in a bilingual context. It is aimed at investigating potential mechanisms driving divergences in heritage language grammars as compared to the “baseline monolingual standards.” For the purposes of the study, 95 bilingual and monolingual children and adults were compared. Bilinguals residing in Israel acquired Russian from birth, while the age of onset of Hebrew varied. The participants completed a production task eliciting accusative case inflections. Both child and adult heritage speakers of Russian with early age of onset of Hebrew (before the age of 5) showed divergences in the production of the accusative case inflections as compared to monolingual Russian-speaking controls (adult and child), whereas grammars of Israeli heritage Russian speakers with later ages of onset of Hebrew, after the age of 5, were found to be intact. On the basis of Russian in contact with Hebrew, the study discusses how heritage language grammars differ from the baseline grammars of monolingual speakers and which mechanisms are associated with heritage language ultimate attainment. The effects of the age of onset and cross-linguistic influence from the dominant societal language are discussed as potential factors affecting the acquisition / maintenance of linguistic phenomena in heritage language grammars.