Background: Historically, pediatric medicines are developed after adult trials are completed, even when identical drug targets and disease similarities exist across the populations. This has resulted in significant delays in the authorization of medicines for adolescent use, limiting access to beneficial drugs. This study sought to understand how adolescent inclusion in adult trials is positioned in regulatory guidance documents as they set critical expectations for trial design and regulatory decision-making. Methods: This study utilized a qualitative analysis approach. Guidance documents were identified via Food and Drug Administration and European Medicines Agency websites. Utilizing a blinded adjudication process, the documents were classified as permissive, exclusionary, or silent regarding recommendations about adolescent inclusion in adult clinical trials. A post hoc analysis of similarities and differences between the Food and Drug Administration and European Medicines Agency guidance documents was conducted to assess the possible role of regional pediatric research laws on age-inclusive trial methodologies as well as emergent themes by therapeutic area. Results: In total, 96 Food and Drug Administration (1977 to 2019) and 106 European Medicines Agency (1987 to 2019) guidance documents were identified for analysis. The guidance contained explicit or implicit recommendations supporting adolescent inclusion in adult trials in 32% of Food and Drug Administration and 15% of European Medicines Agency documents, while 14% and 21%, respectively, were found to be exclusionary. A large number of guidance documents were silent regarding the applicability of adolescent-inclusive trial designs (53% and 64%, Food and Drug Administration and European Medicines Agency, respectively). Analysis by therapeutic area revealed the most permissive of adolescent inclusion in Food and Drug Administration guidance for infectious diseases and conditions requiring blood products in European Medicines Agency guidance. A more holistic approach to age-inclusive trial design was identified in disease guidance published by the Food and Drug Administration Oncology Center of Excellence. Discussion: There are many influences on the development and/or revision of regulatory guidance documents. Substantial scientific knowledge and regulatory precedence for the inclusion of adolescents within adult trials are available to inform research approaches. Our study has identified important opportunities for the enhancement of guidance. For example, contextualization of developmental factors influencing adolescent disease progression provides insights into the role of adolescent inclusion. If addressed, guidance documents can facilitate broader acceptance of age-inclusive trial methodologies and accelerate adolescent access to medicines.