The present study explored the effects of interactionist Dynamic Assessment (DA) and age on the writing accuracy of IELTS candidates. This research investigated the existence of an interactional effect between the age of learners and the type of treatment they received. First, 140 IELTS candidates were split into two age groups, adult and young. Each group was divided into two equal smaller groups, randomly assigned to two types of DA. All four groups took a writing pre-test. After a nine-session treatment of interactionist DA in study groups and interventionist DA in control groups, all groups took a writing post-test. The data collected during the post-test were analyzed by two-way ANOVA and compared with the pre-test data. The findings showed that implementation of interactionist DA significantly improved the learners' writing accuracy while the learners' age had no significant effect on their writing accuracy. With respect to the study findings, EFL teachers could use interactionist DA to enhance the accuracy of learners' writing skills by providing more opportunities for the learners to interact.