Board in their assigned boarding position. If they don't get sequential numbers they can board together at the higher (less good) boarding position.
If you or another adult is available, you are able to walk with them to the gate and assist them. Request a gate escort pass at the check-in counter. IMO most 15 year olds should have no problem navigating an airport. Ensure they have some money for food and a print out of their itinerary in case they need to ask questions. Any Southwest or TSA employee can assist them.
... View more