In response to the growing interest in evaluating young learners' foreign language (FL) performance, this study aims to deepen our understanding of young learners' developmental differences in interaction during task-based paired-language assessments. To examine age effects separately from the effect of general language proficiency, we analysed learners' interaction in both their first language (L1) and FL. We observed the students as they engaged in an assessment consisting of a set of decision-making tasks with their peers. After each task, the students self-assessed their performance. We found differences in interactions across grade levels in both L1 and FL with respect to holistic interactional patterns, topic development, and turn-taking patterns. The students' understanding of communicative interactive tasks also appeared to differ between grade levels, perhaps reflecting their experiential differences with communicative tasks in class. The paper concludes with implications for implementing task-based paired-language assessments for young learners.