The United Airlines unaccompanied minor fee is an additional fee that customers pay when they want their child to travel alone on a flight. The fee varies depending on the length of the flight and is charged per one way. An unaccompanied minor defined by United would have to be between the ages of five and 14 years old. So, a passenger that is five years old flying without an adult at least 18 years old, would be considered an unaccompanied minor. The same goes for a 14 year old travelling alone without an adult at least 18 years old.