The present paper examines the lexical profiles of children and adult EFL learners in an attempt to untangle the roles of age and hours of exposure and their effect on lexical knowledge. Both age and exposure time have generally been acknowledged as relevant factors influencing foreign language acquisition, and vocabulary learning specifically. Participants in the study, who share the same amount of exposure to the target language but differ in age, were asked to write an informal letter introducing themselves. Their letters were scrutinized and mean production of tokens and types was found together with word frequencies and some lexical inconsistencies. In addition, the children participating in the study were enrolled in a CLIL program to match their hours of instruction with those of the adults. Their lexical profiles are characterized by fewer tokens and types and words of lower frequency related to the fields of school, home and science, the CLIL subject. They also use more cognates and commit more lexical inconsistencies than adult learners, who in turn produce more general words in longer letters. We interpret our results in light of the relevant role of age and instructional program.