I think the age of the plane is a canard. Look at DAL (Airbus and Douglas) and AA with USAir's old guppies. If the planes are maintained, they can still make money. And if gas remains cheap, this is especially true. During my last PC the Check Captain said the UA planes with the worst dispatch reliability were the new guppies. And given that UA is sorely in need of narrow lift these days, I think the Bus fleet is pretty safe for a while.