Final answer:
JetBlue is the newest airline among Southwest, Frontier, JetBlue, and Spirit, having been founded in 1998.
Step-by-step explanation:
The newest airline among Southwest, Frontier, JetBlue, and Spirit is JetBlue. It was founded in 1998 and is the most recent addition to the group. Southwest, on the other hand, was founded in 1967, making it the oldest among the four companies.