But the 35-year-old airline is feeling its age, with labor costs rising along with expenses related to maintaining a fleet of more than 400 planes. As its fuel-hedging benefits begin to erode, Mr. Kelly must continue to tighten spending while maintaining the famous warm-and-fuzzy Southwest culture that has generated some of the most loyal employees in the industry. His ability to do that will be severely tested next year when Southwest must negotiate a new labor contract with its pilots union. The airline’s financial strength and its long, unbroken string of quarterly profits could make it harder to keep a lid on salaries and benefits.