This paper reports the professional journey of an English as a foreign language (EFL) teacher from teaching older learners to teaching younger children at a primary school, and the impact of mentoring on the teacher in facilitating the transitioning process. The participant is a Turkish native-speaker male English teacher with 23 years of teaching experience. He participated in the mentoring programme, which was organized as a collaborative action research teacher development project, and implemented by the author of the present study. During this process, the participant completed three cycles of action research. For each cycle, he identified a problem and/or any aspect of teaching he wished to improve, designed an action plan, applied it in his Grade 2 English classes, reflected upon his action, and documented his action research. He was also interviewed to gain additional insight into his experiences. Qualitative inductive analysis was used to analyse the interviews and reflective writings. The findings suggest that the mentoring process led to an increase in the teacher's self-efficacy in young learner pedagogy and teaching performance, helped him socialize into the community of young learner teachers, and gain teacher-researcher identity, which is perceived to smooth his transition into teaching a younger age.