This article examines how international differences in age-grade distributions and grade effects contribute to science scores among 27 Organization for Economic Cooperation and Development countries. As shown in the 2006 Program for International Student Assessment, countries vary substantially in the grade distribution of 15-year-olds. The costs of being in a grade lower than one's 15-year-old peers are higher in some countries (e.g., Greece and Spain); conversely, the benefits of being in a higher grade are greater in others (e.g., Australia and Luxembourg). Although the United States has relatively greater percentages of 15-year-olds in higher grades, US 15-year-olds receive relatively fewer benefits from higher-grade membership. (Contains 3 figures, 4 tables, and 17 footnotes.)