This study has been conducted with the aim of analyzing sex differences in fluid intelligence of schoolchildren of different ages. Three samples of schoolchildren participated in this study. Fluid intelligence was measured by Raven’s Standard Progressive Matrices (SPM), Raven’s Standard Progressive Matrices Plus (SPM Plus), Raven’s Advanced Progressive Matrices (APM), Culture fair intelligence test, Scale II, (CFIT), D-70 and Problem solving test, (PT). Results obtained in our study indicate that at ages of 12.6 and 16 effect sizes of sex difference in performance on tests of fluid intelligence were small. At age of 17.2 boys scored almost one standard deviation higher than girls. Gender stratification could have led to different experience of boys and girls, which, in turn, could have caused investment of cognitive resources in different domains.