Since , up to 12 years old passengers are considered children , passenger 13 y/o and olderwill be considered an adult.
Even being an adult , passenger 13-17 cannot travel without a passenger more than 18.
It is not possible to make a booking whith just one passenger under 18 years old.
You will need to contact customer service to make the booking and link with an adult's booking (more than 18). Monday - Friday 9:00 – 5:00 PM (U.S.A GMT )