This study explains the development of science, technology, engineering, mathematics (STEM) interest among elementary and middle schoolchildren. The cohort longitudinal design was applied, starting with three cohorts of students--fourth (10 years), fifth (11 years), and sixth (12 years) grade--followed for three consecutive years. A total of 947 pupils responded to general and specific STEM interest measures. The results show that the level of STEM interest of children is generally low. Gender differences in STEM interest in favor of boys are apparent in all STEM areas, except science. The observed gender gaps in interest over time are constant, except for a small increase in gender difference of engineering interest. The average rate of change of STEM interest over time is mostly insignificant. Large interindividual variability of interests' scores and slopes indicates that the level of STEM interest and its change over time are highly individualized phenomena.