This study explores dual language acquisition according to three interrelated factors which have been found to account for individual differences in bilingual development. These are child-internal, proximal and distal factors. This five-year longitudinal case study investigates the complex interplay between language exposure, language skills and language and cultural identity construction in three French-English bilingual adolescents (from age 10 to 14), from three different home language backgrounds (French dominant; English dominant; both French and English), attending the same bilingual programme in France. Data were collected annually. Parent questionnaires provided information on the children’s exposure to French and English from birth, within the family and in school and other social environments. Semi-structured interviews with the children explored their current language exposure and their language practices with close family and friends, and in language-based activities, such as reading. Language skills were assessed in both languages through (1) a narrative task evaluating lexical diversity and grammatical accuracy, and (2) a standard receptive vocabulary task. Identity construction was explored through semi-structured interviews and a language portrait activity. Our findings showed, first, that higher exposure to a language at home and school did not necessarily align with higher level skills in that language. High-level skills were also observed in the language where exposure was quantitatively lower, but qualitatively rich. Secondly, despite higher exposure to one language, children sometimes identified more with the language and culture they were exposed to less. We highlight the importance of exploring children’s exposure and language biographies in depth to distinguish the sources and types of exposure received from birth. We also show the impact of children’s agency on their language investment and language development.