Language profiles of individuals with Down syndrome (DS) reveal a pattern of heterogeneous abilities, with receptive vocabulary exhibiting strengths over receptive grammar, and expressive language lagging behind. Little is known about inferential abilities in this population, in either children or adults, despite inferencing playing a pivotal role in language comprehension. Inferential abilities are particularly relevant to the successful understanding of narratives, as story plots combine explicit (factual) and implicit (inferential) information. This study investigated inferential abilities in 26 English-speaking adolescents and adults with DS (age: 13-43, M=22.9 years) compared to 23 young vocabulary-matched typical controls (age: 4-11, M=6.96 years). Inferencing was assessed through a narrative comprehension task, which targeted understanding of story characters' goals and internal states (ISs). Participants with DS showed poorer comprehension of inferential questions, across both goals and ISs, with vocabulary level and receptive grammar positively contributing to the comprehension of inferences. Working memory showed a positive albeit non-significant relationship with inferencing ability, while executive functioning skills had no effect. Our results suggest that difficulties understanding, and potentially expressing, inferential information relating to story characters' goals and ISs persevere into adulthood in individuals with DS. Such difficulties are moderated by general verbal abilities and seem driven by poor grammatical skills. We discuss the contributions of verbal and nonverbal abilities to inference-making in Down syndrome, and potential implications for future research.