This study investigates the role of biological age and L2 exposure on the achievement of two groups of Catalan-Spanish intermediate learners of English in secondary school (Group A, Formal Instruction (FI), N = 50; Group B, FI + CLIL, N = 50) regarding receptive and productive L2 skills as well as grammatical knowledge. Learners were matched for hours of exposure (1.330–1.400) in a first comparison, and secondly, for age (13–14 years old). When matched for number of hours of exposure, results confirmed the older learners’ advantage in FL contexts, as non-CLIL students (2 years older) significantly outperformed CLIL learners in listening comprehension and in two measures of writing: accuracy and coordination index. When matched for age, the group with extra L2 exposure (FI + CLIL) was significantly better than the non-CLIL group in reading comprehension and in several dimensions of writing: lexical richness, linguistic and communicative competence. These findings illustrate the language learning potential of a partial CLIL programme in an EFL context. A threshold of 300 CLIL hours may need to be surpassed for CLIL learners to reap the benefits of additional exposure across L2 skills.