The underrepresentation of young adults in legislative bodies in the United States represents a democratic deficit. Nonetheless, little is known about the factors that shape their underrepresentation in this context. In this article, I examine the severity of young adults' underrepresentation in U.S. state legislatures and explore how four institutional mechanisms help to explain representational variations across the states. I demonstrate that 1) young adults in the voting age population are underrepresented in state legislative lower chambers by an average factor of 3, but that some states are better performers than others for the period 2018–2022; 2) a combination of institutional arrangements can positively impact young adults’ descriptive representation: the harmonization of voter and candidate age requirements, increased legislative professionalism, and the use of term limits and single-member districts. This article thus contributes both to our knowledge of young adult representation in a critical case and to the scholarship on subnational politics.