Recognizing that current frameworks for classification and treatment in psychiatry are inadequate, particularly for use in young people and early intervention services, transdiagnostic clinical staging models have gained prominence. These models aim to identify where individuals lie along a continuum of illness, to improve treatment selection and to better understand patterns of illness continuity, discontinuity and aetiopathogenesis. All of these factors are particularly relevant to help-seeking and mental health needs experienced during the peak age range of onset, namely the adolescent and young adult developmental periods (i.e., ages 12-25 years). To date, progressive stages in transdiagnostic models have typically been defined by traditional symptom sets that distinguish "sub-threshold" from "threshold-level" disorders, even though both require clinical assessment and potential interventions. Here, we argue that staging models must go beyond illness progression to capture additional dimensions of illness extension as evidenced by emergence of mental or physical comorbidity/complexity or a marked change in a linked biological construct. To develop further consensus in this nascent field, we articulate principles and assumptions underpinning transdiagnostic clinical staging in youth mental health, how these models can be operationalized, and the implications of these arguments for research and development of new service systems. We then propose an agenda for the coming decade, including knowledge gaps, the need for multi-stakeholder input, and a collaborative international process for advancing both science and implementation.