Adolescents in Germany eat fewer animal products than their parents, often for sustainability reasons. We investigated how adolescents differ from their parents' generation in sustainability food-choice motives, consumption of animal products, and corresponding behaviors such as advocating for and influencing decisions towards more sustainable family meals. In an online questionnaire, an educationally diverse sample of 500 adolescents (M = 17.9 years, range = 15–20) and 500 adults of their parents' generation (M = 52.2 years, range = 45–60) reported food-choice motives, their own and their family's diet style, how they advocate for sustainable food decisions at family meals (e.g., less meat), and how they influence different steps in family meal planning (e.g., grocery shopping). The two generations did not differ in sustainable food motives and mean consumption frequency of meat and animal products, but adolescents reported three times more often than their parents' generation to never eat meat. At shared family meals they advocated for eating plant-based substitutes (d = 0.27, p < 0.001) and other animal products (β = −0.15, p = 0.02) more often than their parents' generation, but not for eating less meat. Adolescents advocated more frequently for sustainable food decisions at shared meals the more important sustainability motives were to them (β = 0.53, p < 0.001), and the less meat (β = −0.35, p < 0.001) and fewer other animal products (β = −0.11, p = 0.015) they consumed. Adolescents motivated towards sustainability have the potential to impact the family's dietary choices through reverse socialization processes. These findings challenge current theories that suggest only parents influence their children, neglecting the role of adolescents as potential agents of change for improved family and planetary health.