The objective of this paper, in line with the other papers of this special issue, is to show the potentialities of combining intelligence research and cognitive psychol- ogy. The development of intelligence is here addressed from two usually separate perspectives, a psychometric one, and a neo-Piagetian one. Two studies are pre- sented. In Experiment 1, children aged 6, 7, 9, and 11 years (N = 100) were administered two working memory tasks and three Piagetian tasks. In Experiment 2, children aged 8±12 years (N = 207), young adults aged 20±35 (N = 160), and older adults aged 60±88 years (N = 135) were administered working memory and processing speed tasks, as well as the Raven Standard Matrices task. Regression and commonality analyses were run to analyse the age-related variance in the Piagetian tasks (Study 1), and in the Raven task (Study 2). In both experiments, working memory accounted for a large part of the age differences observed, but more so in Study 1 (Piagetian tasks) than in Study 2 (Raven task). It is concluded that working memory mediates the effect of age on fluid intelligence during childhood and during adulthood.