Youth activism for climate justice is inherently intergenerational. Fundamentally, young activists demand urgent action by today's adult power‐holders for the security and well‐being of their own and future generations. Despite intergenerationality being core to the movement, few studies with young activists have explored their views and experiences of intergenerational relations and tensions and how to advance intergenerational solidarities for climate justice. Addressing these critical topics, the present study used in‐depth interviews with young activists (ages 15–17) in the climate justice movement across the US. Themes generated through reflexive thematic analysis centre on: (1) youths' adoption of “next generation” and “last generation” identities, respectively emphasising the heightened climate risks faced by their own and future generations, and the closing window of opportunity to prevent catastrophic climate breakdown; (2) their experiences of hostile and benevolent adultism; and (3) the need for adults to listen to, take seriously, centre, amplify, and—most importantly—respond to youths' demands. They urge adults, particularly those in powerful positions, to use their age‐based privilege, political enfranchisement, material resources, professional status, and decision‐making authority to uplift young people's voices and tangibly advance climate justice through solidarity‐driven intergenerational partnerships and action. Implications for youth‐centred research and policy are discussed.