Student's mental representations of physical concepts are often different from those of scientists. The research aimed to identify and compare mental representations of light propagation time by school children aged 10 (132 subjects), and 14 (109 subjects) years old. This research was conducted through individual interviews in which the students were asked to locate light propagation time in various tasks-experimental situations. The results of this research show that even though the students of two groups face difficulties in understanding light propagation time, as they grow older, they make statistically significant progress in constructing the conception of light propagation time. These findings allow to seek out educational perspectives on the understanding of the conception of light propagation time in organised scholastic environments.