Your child can travel alone by national trains or airplanes starting at the age of five. To travel on a flight with changing aircrafts, the child has to at least seven years old. In local public transport there are different rules. Often the starting age to travel alone is seven. For children aged between five and twelve, there is the possibility to book the journey for an unsupervised child. Children between two and five years can travel by air in the company of their siblings, if these are at least sixteen years old.
If a minor child is not traveling with the parents, but alone or with another adult, you are advised to give your child a Declaration of consent in which the parent or guardian grants permission to travel. A declaration of authorization of the parents you find here: