According to sociocultural theory, dynamic assessment (DA) merges instruction and assessment. This study attempted to compare the effects of interactionist versus interventionist DA on the writing fluency and complexity of adult versus young IELTS candidates in an international language school in Tehran with the age range of 15-48. One hundred and forty students (90 males and 50 females) who had enrolled in the IELTS preparation course in the spring of 2021 participated in this study. The study participants were divided into four groups namely, young interactionist, young interventionist, adult interactionist, and adult interventionist. The participants in all groups were homogenized based on the result of their performance in a standard version of Preliminary English Test (PET). During the eight-week treatment period, the interactionist groups were given writing tests followed by the instructor’s feedback and guidance, while the interventionist groups received instructions and were given writing test samples without qualitative feedback. The pre-test and post-test comparisons among the four groups by Two Way ANCOVA showed that both young and adult individuals in the interactionist groups had a significant improvement in their writing performance compared to those in the interventionist group. In addition, the study revealed the significant impact of interactionist DA vs. interventionist DA on both young and adult participants' scores in both writing fluency and complexity post-test. Likewise, the results showed no statistically significant interaction between the type of assessment and the candidates’ age. The study presents pedagogical implications for teachers, students, and IELTS preparation program designers. پرونده مقاله