Background <p>Self-harm is associated with significant distress in children and adolescents. The objective of this scoping review was to map the age of onset of self-harm in people aged ≤ 18 years alongside the definitions, operationalisation, and research methods used to determine onset.</p> Method <p>Following JBI guidance, this review included studies reporting the age of onset of self-harm in people aged ≤ 18 years in any context. Medline, PsycInfo, Embase, CINAHL Plus, and Web of Science were searched last on 7th May 2025 and supplemented by a grey literature search. Data were subject to basic coding and narrative and graphical presentation.</p> Results <p>A total of 42 studies were included in the review. Age of onset ranged from 9 to 18 years, but most studies reported a mean age between 12 and 14 years. The majority of studies defined self-harm as either suicidal or non-suicidal (85%), with non-suicidal self-harm distributed in favour of a slightly younger onset age. Studies with a younger sample tended to report a younger age of onset. Most studies used cross-sectional methods (81%) and retrospective report (71%) to capture onset age.</p> Conclusions <p>Earlier age of onset is associated with the use of multiple methods of self-harm, self-harm of longer duration and increased frequency. A clear understanding of age of onset of self-harm is necessary to inform clinically relevant research and the timely targeting of developmentally prevention and early intervention strategies.</p>