Despite the persistent gender gap in political knowledge has been largely studied, little is known on its origins: when and how the gender gap does exactly appear? Furthermore, the fact that the gender gap is larger among adults tan among young people has not yet been explained. Building on political socialization research and socioeconomic models in political behaviour literature, this paper proposes that gender differences are enlarged during the transition to adulthood due to inequalities in adult roles and how young men and women live and experience the acquisition of adult roles. We examine the gender gap during the transition to adulthood by means of a unique representative survey of the Spanish population. This survey was exclusively aimed at measuring Spaniards levels of political knowledge and thus counts with a large and varied battery of political knowledge questions. Furthermore, an additional sample of young people allows us to examine young people’s political knowledge after completing specific steps in their transition to adulthood. Findings show that indeed the gender gap in political knowledge is larger among adults in comparison to young people, and that differences between men and women appear during young people’s transitional period. Moreover, our findings indicate that having at least one child is particularly relevant in explaining the appearance of the gender gap. To further comprehend this phenomenon, we present a test of four mechanisms that could explain why women may pay less attention to politics than men when forming a family: 1) time availability; 2) economic resources available; 3) political socialization; 4) temporal withdrawal from the political word.