Good information. Thank you!
How about the age of the Southwest aircraft?
From
Average Fleet Age for Selected U.S. Carriers (Last Update April 2006):
AirTran -- Average Age: 3.7 years / Fleet Size: 108 aircraft
Northwest -- Average Age: 10.8 years / Fleet Size: 266 aircraft
Southwest -- Average Age: 9.4 years / Fleet Size: 445 aircraft
Of courses, averages are just averages.
AirTran has a genuinely young fleet. In addition to Boeing 717, AirTran also has new Boeing 737-700 aircraft (primarily for AirTran's longer routes).
In the case of Northwest, their DC-9-30 fleet has a much older average age than the rest of their fleet.
Southwest has aircraft ranging in age from early 737-300 models to brand new 737-700 models. Southwest has retired all of their 737-200 aircraft. I was on a Southwest roundtrip a couple of weeks ago. One aircraft was older and one looked brand new -- but both were very clean inside, with leather seats.
Properly maintained aircraft can fly for many decades. The primary reason for retiring older aircraft is because newer aircraft can be operated more efficiently, with lower fuel costs and maintenance costs per passenger-mile. But new aircraft cost a bundle upfront, so the airlines have to perform a balancing act between operating costs and aircraft purchase/lease costs.