The conditions vary slightly depending on the type of ticket or card account in question. Here you can read more about the conditions for single tickets.
Your right to travel is dependent on there being enough space.
Changes and claims
Single tickets cannot be changed and are not refundable.
Pay the correct fare and retain your ticket
You are classed as a student and travel at a reduced rate until you reach the age of 20. Request student fare when you buy your ticket. Children under the age of 7 travel free of charge when accompanied. Children of 6 years of age and above may travel alone but must have their own ticket or card. Keep your ticket during the entire trip.