Advantages and disadvantages of an older fleet
With that being said, there are a number of clear disadvantages to an older fleet. Obviously, worse fuel efficiency is going to be a big extra cost for the bottom rung of airlines, such as Delta Air Lines and Allegiant, which are running, by far, the oldest fleets.
Passenger comfort and amenities could also be affected as a number of older airplanes simply don't have in-flight entertainment systems. Airlines like Alaska Air have done their best to counter this with its digEplayer, a small, portable, downloadable entertainment device which flight attendants pass out during the flight and collect toward the end of a flight for a nominal fee. Not surprisingly, though, a number of the worst performers in overall customer satisfaction also have some of the oldest fleets.
On the flip side, operating an older fleet can have its advantages -- just ask Allegiant. Although Allegiant pays more in fuel costs than its peers for an equivalent amount of miles flown, it also pays far less out of pocket to acquire used planes. This difference has allowed Allegiant to remain one of the few airlines that's net cash positive, and gives it the opportunity to focus on the deep-discount niche where consumers are looking for a no-frills flying experience as long as the flight is cheap.