"Designated companions under the age of 18 can accompany Companion Pass holders on international flights; however these passengers cannot be booked online. Please contact us at 1-800-I-FLY-SWA (1-800-435-9792) to book a companion under the age of 18 for international travel."
Poorly written stuff from WN again. Certainly ambiguous.
Should have been "If the designated Companion is under 18, the CP holder must call to make the reservation."