Understanding proportions is a time-intensive process that does not come cheap during late childhood and early adolescence. It is fostered by learning experiences in which students have opportunities to explore, discuss and experiment with situations involving proportions. Children must undergo many informal learning opportunities before they can gain from direct instruction on proportional reasoning. In this study, we aimed to determine whether physics curricula focusing on the concept of density prepares students for learning from a curriculum on proportional reasoning. A 2x2 design with the factors “physics curricula” (with, without) and “concept used to introduce proportional reasoning” (speed, density) was applied to 253 children from 12 classrooms at the beginning of grade 5. We expected the “density, with physics curriculum” group to outperform the other three groups. However, only the students who scored in the highest quartile on an intelligence measure gained from the prior knowledge they had acquired through the physics curricula. The results show that curricula on proportional reasoning are worthwhile for all students in early adolescence. However, more capable students can boost their proportional reasoning if they have the chance to acquire prior knowledge through a physics curriculum.