Age is a legally consequential attribute in forensic contexts, particularly for the protection of children and in the administration of criminal and civil law. This chapter outlines the normative relevance of age thresholds in Europe with reference to Portugal—where criminal responsibility begins at 16 years and civil majority at 18 years—and explains why accurate, standard-based age assessment safeguards due process and children’s rights. We distinguish chronological, biological, and estimated age, emphasising that forensic age opinions ultimately address a normative construct (chronological age) inferred from biological indicators. For living individuals—especially undocumented, unaccompanied asylum seekers—ethical and legal principles apply: best interests of the child, benefit of the doubt, informed consent, non-maleficence, beneficence, justice, and adherence to ALARA when ionising imaging is used. We review bone and dental approaches for sub-adults, summarising variability and accuracy, and argue that teeth—being less susceptible to environmental variation—often provide higher precision. Methodological quality requirements are highlighted (sample verification, sex stratification, population specificity, transparent staging and reference tables). We briefly introduce widely used atlases and scoring systems (e.g., Greulich–Pyle, Tanner–Whitehouse, Schour–Massler, London Atlas, Demirjian, Haavikko; third-molar schemes), and the role of clavicular staging in late adolescence. Throughout, we caution against conflating intrusive with invasive procedures and against substituting psychological or interview-based “age” for legally relevant chronological age in the initial determination. The chapter advocates harmonisation and interoperability of staging vocabularies to enhance reliability, reproducibility, and legal admissibility of age assessments.