This study reveals hitherto overlooked effects of age of onset (AO) in immersive school contexts, using multiple measures over time so as to focus on fluctuations and trends in individual data. The second language (L2) English development was studied in 91 children who received 50-50 content and language integrated learning (CLIL) instruction in German and English and varied in their AO (5, 7, or 9). Data collection occurred 4 times annually for up to 8 school years (ages 5-12), via oral and written production tasks, motivation questionnaires, and interviews. Meaningfully integrating quantitative analyses (GAMM) and qualitative analyses, the study focuses not only on the process itself and on quantification of change but also on the underlying environmental and psychological reasons for change. Results reveal that (a) slightly later CLIL beginners (AO 7) turn out to show similar L2 development to that of the earlier beginners (AO 5), (b) besides external states and events, many internal states at any given moment contribute to significant L2 growth, (c) learners show significant improvement in the last 2.5 years of primary school, starting from age 10, and (d) there are significant differences between L2 oral and written performance in terms of height and shape of learner trajectories across (pre)primary school.