In order to give informed consent for a medical procedure, a patient must be legally competent to do so. Every adult (i.e., those 18 years of age and over) is presumed to be legally competent, unless and until determined otherwise by a court of law. Minors are not considered competent under law, though there are certain health care decisions that they may be allowed to make. For example, various state laws allow minors access to treatment for substance abuse, testing and treatment for HIV and other sexually transmitted infections, abortion and contraception, and mental health concerns, without parental permission or notification. Emancipation may also render certain groups of minors (e.g., those who are married, parents, or on active military duty) or certain individual minors, legally able to make health care decisions.

Surrogate decision makers make decisions for incompetent patients. Competence is a legal determination, and is not the same as decision-making capacity. A minor, for example, may actually have developed sufficient capacity to make sound decisions, while still being legally incompetent.