Do individual differences in personality traits become more or less pronounced over childhood and adolescence? The present research examined age differences in the variance of a range of personality traits, using parent reports of two large samples of children from predominantly the USA and Russia, respectively. Results indicate (i) that individual differences in most traits tend to increase with age from early childhood into early adolescence and then plateau, (ii) that this general pattern of greater personality variance at older childhood age is consistent across the two countries, and (iii) that this pattern is not an artefact of age differences in means or floor/ceiling effects. These findings are consistent with several (noncontradictory) developmental mechanisms, including youths' expanding behavioural capacities and person–environment transactions (corresponsive principle). However, these mechanisms may predominantly characterize periods before adolescence, or they may be offset by countervailing processes, such as socialization pressure towards a mature personality profile, in late adolescence and adulthood. Finally, the findings also suggest that interpreting age trajectories in mean trait scores as pertaining to age differences in a typical person may sometimes be misleading. Investigating variance should become an integral part of studying personality development.