Children who did not have their sixth birthday yet can travel free. For safety reasons this category of children can travel only when accompanied by a person older than 15 years of age.
Children who had their sixth birthday but did not have their sixteenth birthday yet can travel for a reduced fare. It isn't necessary to show any evidence about the age of the child. In case of any uncertainty about the age of the child you can be asked to clarify the age by a trustworthy document, for example an insurance card.