Uncertainty persists regarding the post-childhood trajectory of syntactic acquisition of persons with Down syndrome (DS). In some studies, asymptote is reached in the early teens, whereas others find syntax continuing to develop at least into early adulthood. This study addressed the issue using a cross-sectional approach. Receptive syntax and vocabulary were tested in 62 children, adolescents and young adults with DS matched on chronological age and cognitive level with 62 participants with intellectual disability (ID) of undifferentiated etiology. On both tests there were significant effects of chronological age and diagnosis, but the chronological age × diagnosis interactions were nonsignificant. We concluded that comprehension of vocabulary and syntax does not asymptote prematurely in individuals with DS relative to those with other forms of ID.