Vocabulary proficiency is a key predictor of reading development. However, vocabulary proficiency in school-age children is rarely assessed, especially in languages other than English. Moreover, because reading development differs depending on home language and gender, it is likely that these factors also influence the development of vocabulary proficiency. Here we report Finnish vocabulary proficiency of school-age children, examining its relationship with grade, gender, and home language. We utilize d-Lexize, a vocabulary test based on visual lexical decision, which we adapted from a previous test for adult L2 speakers. The test assesses vocabulary knowledge by accuracy and lexical retrieval speed through reaction time. Approximately 27,000 school children were tested in three experiments using different versions of d-Lexize. All experiments consistently show that vocabulary proficiency improves progressively from 3rd to 9th grade. The results also reveal an emerging gender gap: whereas girls perform equal to boys in the early stages, they exhibit a more extensive vocabulary and faster lexical retrieval in the later grades. Furthermore, the tests show that pupils from Finnish-only homes consistently outperform those from non-Finnish or mixed-language homes, with this gap widening over time. These results highlight the significance of language exposure and sociocultural factors during vocabulary development.