This paper quantifies the learning gain that accrues to 15-year-old students over one year of schooling in 18 countries and economies, where the cohort eligible to sit the OECD Programme for International Student Assessment (PISA)ISA test overlaps with two distinct school cohorts. School-entry regulations are used as an exogenous source of variation for grade levels in an instrumental- variables framework. The focus on the joint effect of schooling and age, together with (local) linearity assumptions, make it possible to account for differences in school-starting age across students who are expected to be in different grades. On average, students' test scores increase by about one-fifth of a standard deviation over a school year. While estimates of the grade gain for individual countries and economies come with wide confidence intervals, this study also shows the annual learning gain of students around the age of 15 tends to be larger in high-income countries compared to middle-income countries.