The aim of this study was to identify promising endophenotypes for intelligence in children and adolescents for future genetic studies in cognitive development. Based on the available set of endophenotypes for intelligence in adults, cognitive tasks were chosen covering the domains of working memory, processing speed, and selective attention. This set of tasks was assessed in a test-retest design in children and in adolescents. Working memory could be measured reliably using the n-back task and correlated with intelligence in both age groups. For processing speed, assessed with the big product-inspection time task and reaction time on the flanker task, test-retest reliability was good in both age groups, but processing speed only correlated significantly with intelligence in children. Selective attention, i.e., the effect of incongruent flankers on RT and accuracy, showed low reliability and neither correlated with intelligence in adolescents nor in children. Thus, working memory seems a promising endophenotype for intelligence in both children and adolescents. Inspection time and measures of selective attention based on the flanker task do not seem very promising endophenotypes for intelligence in these age groups.