To date, the vast majority of research on the linguistic abilities of heritage speakers has focused on young adults whose heritage language is no longer developing. These adults began their journey as bilingual children acquiring the heritage language with the majority language simultaneously since birth or sequentially, as a second language. If longitudinal studies are not always feasible, linking research on the structural development of bilingual pre-school children with research on young adult heritage speakers adds a much needed perspective to understand the initial state and the end state of heritage language development. The purpose of this study is to connect the beginning of heritage language development with its ultimate attainment by comparing the expression of subjects in Spanish in 15 school-age bilingual children and 29 young adult heritage speakers, all of them simultaneous bilinguals with English as the dominant language and Spanish as the weaker language. The oral production of null and overt subjects by child and adult heritage speakers was compared to that of age-matched monolingual speakers in Mexico (20 children, 20 adults). To provide a wider context the study includes a group of 21 adult immigrants, who could also potentially influence the input to the heritage speakers. The results confirm that discourse pragmatic properties of subject expression in Spanish are vulnerable to incomplete acquisition and permanent optionality in child and adult bilingual grammars.