There are no Belgian or international procedures that lay down rules on parental authorization for the travel of minors. But minors travelling alone remain under the responsibility of their parents, and therefore they have to judge whether their child can make a journey alone taking into account the possible dangers. However, it is recommended that you provide written authorization if your child is traveling alone or with others apart from you and your parents. To avoid any unpleasant surprises, ask about the additional documents required for children traveling alone or with one of their parents, at the embassy or consulate of your destination country, as well as with your airline. https://diplomatie.belgium.be/fr/Services/voyager_a_letranger/informations_supplementaires/voyager_avec_des_mineurs
For air travel, it will depend on the airline. Children can travel alone and will be usually taken care of by the airline. However, the age from which the child can travel alone accompanied by a staff member and the age from which he can travel alone without being taken care of by the staff of the company varies.