The increasing prevalence of adults’ prejudice against adolescents on the Internet could cause opposing attitudes and even online aggression against teenagers. Yet, such age discrimination is less challenged compared to other social biases in cyberspace. Employing a social identity approach, this study aims to explore how features of online communication, teen-related personal experiences, and the dual identity of adult aggressors influence online moral disengagement (OMD), motives, and behaviors for cyber-aggression. We conducted an online survey of 767 Chinese adults in May 2023. The results demonstrated that perceived controllability of online speech, negative teen-related online experiences, and generation-based bias were positively associated with OMD. Next, adult respondents’ self-serving motives positively predicted moderate and severe cyber-aggression, while educational motives only increased moderate cyber-aggression. Mediation analysis revealed that generation-based bias was the sole significant factor that amplified OMD, self-serving motives, educational motives, and cyber-aggression behaviors. In contrast, the effects of lifestage-based bias were insignificant. Our findings yield insights into individuals’ moral transgressions in digital environments and shed light on the dynamics of the identity of adult aggressors in age-based discrimination. This study suggests that empathy from adults and age-appropriate prevention by online platforms are crucial to address online aggression against young generations.