Young adults remain significantly underrepresented in legislative institutions across the United States (U.S.), both at the national and subnational levels, contributing to a democratic deficit that undermines trust, engagement, and legitimacy. This deficit is especially consequential for young people who desire representative institutions that understand and advocate for their interests. In this chapter, we examine substantive representation of youth in the U.S. state legislatures. Focusing on six states—Arizona, Georgia, Hawaii, Michigan, New Hampshire, and Oklahoma—we use three complementary data sources to analyze the alignment between young constituents’ policy priorities and the legislative behavior of young adult state legislators to address three overarching questions: (1) What are the issue priorities of young people and young lawmakers in the U.S.? (2) Do young legislators advocate for the issues most important to young people through the bills they introduce? (3) Do factors such as party affiliation, gender, and institutional status (tenure, ranking) shape the extent to which young lawmakers represent youth policy priorities? Our findings suggest that young legislators share issue priorities with their young constituents and actively introduce bills that address these concerns. However, representation varies across issue areas, by party affiliation, race/ethnicity, and by gender. This chapter contributes to broader debates on institutional responsiveness, political inclusion, and the health of representative democracy. Furthermore, it highlights the critical yet under-examined role of subnational governments in promoting substantive representation.