ABSTRACT Ageism against older adults has been well studied, yet adolescents also experience ageism in pervasive and harmful ways. In this article, we describe anti-youth ageism as a system of oppression that encompasses negative stereotypes, prejudices, and discrimination against adolescents that uphold power hierarchies and marginalize young people based on their age. Drawing from interdisciplinary theory and research, we examine adolescents' experiences of anti-youth ageism at interpersonal, cultural, and institutional levels, and consider the ways anti-youth ageism is internalized. Across many levels, anti-youth ageism is understood in concert with other systems of oppression such as racism and cis-heterosexism. The field needs a new wave of anti-oppressive developmental science to understand the multilayered, intersectional manifestations of anti-youth ageism and the impacts of anti-youth ageism on various domains of development. Research can help foster the creation of intervention strategies to reduce harm to adolescents and their development.