This study explores how learners across school levels engage with a digital extensive reading (ER) environment by comparing upper elementary and lower secondary EFL learners (N = 100). Both groups participated in a four-week multimedia e-book reading program designed to support ER through multimodal features and teacher guidance. Using a mixed-methods design with pre-post reading measures, surveys, and classroom observations, the study examined changes in reading fluency and affective engagement. Results showed that both groups improved in fluency and affective engagement, although patterns differed by school level. Confidence and interest improved in both groups, while motivation increased only among secondary learners. Reading speed improved across groups, whereas comprehension gains were significant only for elementary learners. Elementary learners’ progress reflected teacher-mediated scaffolding, whereas secondary learners showed greater self-regulated engagement. Correlation analyses showed that confidence was most strongly associated with reading fluency. These findings suggest that digital ER is most effective when instructional support and digital affordances align with learners’ developmental needs.