The influence of prior knowledge and cognitive development on the effectiveness of iconic representations in science visualizations was examined. Middle and high school students (N = 186) were given narrated visualizations of two chemistry topics: Kinetic Molecular Theory (Day 1) and Ideal Gas Laws (Day 2). For half of the visualizations, iconic representations of key information were added. Results indicated a main effect of prior knowledge on learning in Day 1. In Day 2, a three-way interaction was found between prior knowledge, age group and icons: icons were effective for all middle school students and for high school students with low prior knowledge, but were not effective for high school students with high prior knowledge. These findings indicate that the expertise reversal effect can be mediated by cognitive development and other factors, not just domain specific prior knowledge.