Despite the growing body of studies on action research (AR) in second/foreign language (L2) education, little is known about teachers' beliefs about AR among different age groups. Hence, this study investigates whether L2 teachers' beliefs about AR vary by considering teachers' age. The Inventory on Teachers' Beliefs about Action Research (ITBAR) is a specifically designed questionnaire to investigate the matter and was distributed to 157 Iranian ELT teachers. Then 120 teachers were randomly divided into three age groups, namely starters (20-34 years), middle-aged (35-49 years), and seniors (50+). A one-way ANOVA test was used to test whether the difference among the groups was significant. The findings showed that teachers can experience age effects in the demanding teaching profession. According to the data, younger teachers had the highest mean scores for their beliefs about AR in comparison to their middle-aged and senior colleagues. This might be attributed to the different perspectives of these teachers toward professional development, the role of pre-service teacher programs, and the familiarity of senior teachers with the limitations of conducting AR in real classroom contexts. It is then suggested that designing collaborative AR projects between beginning and senior teachers and institutional support might lead to more fruitful and positive results.