This chapter considers the tension that sometimes arises between (a) children’s right to protection of their security of the person i.e. through ‘best interest of the child supervision’ of the court and (b) the child’s right to agency over their bodies (as an aspect of the constitutional liberty right). That tension is considered in this chapter in the context of (a) the denial of physician-assisted death in Canada to terminal, suffering children (a medical procedure available to eligible adults in that State), (b) children’s refusal of life-saving medical treatment and the issue of justification for applying or not applying a judicial override to the child’s decision, and (c) the contested right of transgender adolescents in several US states regarding their access to gender-affirming medical care. Also considered is age discrimination in law in the assessment of child competence to make medical decisions in both consent and refusal of medical treatment situations. In UK law this is considered in terms of the fact that children over aged sixteen are presumed legally competent, a potentially rebuttable presumption in the individual case, while children under age sixteen are presumed legally incompetent, a rebuttal presumption also based on the specifics of the particular case.