Background: Language exposure is known to be a key factor influencing bilingual vocabulary development in typically developing (TD) children. There is, however, a lack of knowledge in terms of exposure effects in children with developmental language disorder (DLD) and, especially, in interaction with age of onset (AoO) of second language acquisition. Aims: In the Helsinki longitudinal SLI study (HelSLI), we investigated the receptive and expressive second language (L2) vocabulary performance and cross-sectional vocabulary development of sequentially bilingual children with TD and DLD in order to resolve whether the groups could be differentiated based on their vocabulary performance. More importantly, we examined the effects of AoO and exposure in the vocabulary performance of these two bilingual groups. Methods & Procedures: A total of 70 children with DLD from a hospital clinic and 82 with TD from kindergartens were recruited. Children were 3-7 years old with different AoOs for the L2 and varying degrees of language exposure. Multiple regression analysis was used to compare the groups in two receptive and three expressive vocabulary tests while considering the effects of AoO and language exposure. Outcomes & Results: Children with TD outperformed children with DLD in both receptive and expressive vocabulary measures. Exposure predicted vocabulary but AoO did not. The effect of exposure was different in TD and DLD groups in receptive but not in expressive vocabulary. Additionally, the interaction of exposure and AoO was found for receptive vocabulary, but similarly in both groups. With increasing exposure, a difference in performance between the groups became more notable in receptive vocabulary compared with the expressive vocabulary. Conclusions & Implications: Bilingual children with TD and DLD can be differentiated by using L2 vocabulary tests when exposure is taken into consideration. Non-significant AoO effects in 3-7 year olds suggest flexibility in terms of when to start L2 immersion. However, exposure is important, and especially children with DLD would need a substantial amount of it relative to their TD peers, so that they would not fall even further behind over time. Differences in benefiting from exposure in receptive mode might offer clinicians and kindergarten personnel an insight for evaluating challenges in bilingual development. Severe challenges in vocabulary development in the DLD group also call for both individually targeted small-group activities for learning words as well as strategies for strengthening vocabulary in various environments and everyday life situations.