The mathematics achievement discrepancy between girls and boys, with its subsequent occupational consequences, is an issue that has received considerable attention in the literature. It is often referred to as the ‘math-gap’ and favours boys. A major component of the explanation of this gap resides in determining its age of onset. We analyse here data from more than 10,000 (cross-sectional study) and 2000 (longitudinal study) French students aged 4–7 years, tested in the framework of the Etude Longitudinale Française depuis l'Enfance (ELFE). The results allow to precisely determine the age of onset, since the gender difference, non-existent (or even slightly in favour of girls) in kindergarten (4–5 years), is clearly in favour of boys in first grade (6–7 years). They could therefore provide an important element in the controversial debate on the origin of gender-differentiated performance in mathematics.