The present study longitudinally explores regressive phonological cross-linguistic influence (CLI) in seven adolescents (aged 12–13) and seven adults (aged 21–39) by examining voice-onset time (VOT) of /p,t,k/ in their first, second, and third language (L1, L2, and L3, respectively). All participants had the same language combination (L1 German, L2 English, L3 Polish) and were recorded completing a range of production tasks in all three languages four times over the course of the first year of

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... 3 learning. The scope of previous research on phonological CLI is thus broadened in two ways: (1) by tracing the development of all languages upon the arrival of a new language in a multilingual's system longitudinally, and (2) by investigating CLI patterns in two age groups when input and learning environment are comparable. Previous L2 age studies have mostly only made retrospective assumptions about (target) language development, so that longitudinal data, including the entire language repertoire of multilingual speakers, are needed to substantiate claims made in that regard. For the adolescent group, significant changes to both their L1 and L2 over time were found, while the adults' background languages remained relatively stable on the group level. However, for both groups, much individual variation was uncovered.