Numeracy and literacy skills have become increasingly important in modern labour markets. The large gender differences that several studies have identified have therefore sparked considerable attention among researchers and policy makers. Little is known about the moment in which such gaps emerge, how they evolve and if their evolution differs across countries. We use data from large-scale international assessments to follow representative samples of birth-cohorts over time, and analyse how gender gaps in numeracy and literacy evolve from age 10 to age 27. Our results suggest that, across the countries examined, males' advantage in numeracy is smallest at age 10 and largest at age 27. The growth in magnitude of the gender gap is particularly pronounced between the age of 15 and 27. Such evolution stands in sharp contrast with the evolution of the gender gap in literacy, which is small at age 10, large and in favour of females at age 15, and negligible by age 27.