The general objective of this dissertation is to verify the possibility of applying a different legal regime than the proposed by the Civil Code to existential legal situations involving decision making of children and adolescents, especially those that involve decision about their own body. To this end, there are three specific objectives to be developed. The first is the presentation of the theory of legal capacity and the fundamental rights of personality in order to understand the perspective of legal provision of fundamental rights of personality. The second objective is to investigate the human dignity as autonomy to verify the application of theories to support the private autonomy of the child and adolescent in existential legal situations. Finally, the third objective is to discuss the ability to act of children and adolescents in existential legal situations from three specific cases. As a general conclusion it is sustained the application of realist theory s ability to act, bringing together concepts of ownership and legitimacy when children and adolescents meet the requirements to be subject of consent, and that the criteria of validity of the consent must be observed. It was also concluded that the realist theory of capacity to act should be applied to give voice and dignity to children and adolescents when they involve existential legal situations of reversible effects. So that the latter conclusion has created three areas of application of the theory in order to upgrade it to respect the process of maturational development of children and adolescents and thus ensure their dignity.