The current study investigated differences in lexical knowledge of Arabic learners whose age of onset (AO) of exposure to English medium instruction (EMI) was at the elementary, secondary, and tertiary educational levels. Ninety undergraduate students enrolled in a public university in the UAE took part in the study. Data collection involved a background questionnaire, a vocabulary size task, and a vocabulary depth task. Using the background questionnaire, the participants were separated into the early, middle, and late learners -- those exposed to EMI in elementary, secondary, and tertiary levels, respectively. The results revealed a significant multivariate main effect for the AO to vocabulary knowledge. The post hoc analysis confirmed a significant effect for vocabulary size only; no such effects were observed for vocabulary depth. Theocratical, methodological, and pedagogical implications are discussed.