do you really think Delta has a "pretty new fleet"? I can tell you their average plane age is at about 12,5 years, not very good compared to United (ca. 9,8), American (ca. 10,0) and especially Southwest (ca. 7,8).
Beside the 727 and L1011 Delta has many old 767(-200) and 757, as long as many 737-200 (Adv) so that explaines the high average age.
European and Asian airlines do much more for young fleets, Lufthansa, KLM, Swissair, Singapore Airlines and Cathay Pacific for example, all their ages are under 8 years!
Well but I see American airlines have much bigger fleets, so it is not very easy to keep a fleet of more than 400 or 500 aircraft brand new.
I think Northwest is one of the worst in the field of fleet age. Their average age is more than 19 years!!! Although they have more than 50 A320, the many many DC-9s keep the age high.
I hope they will order the A318 soon, not the B717 which is not more than a re-named, old-standart DC-9-30.
Interested in talking more about that great aviation? Write me to Germany, it would be a pleasure.
Me & You & a Plane Named Blue...