UAL just announced today the earlier retirement of 25 727s next year. They also announced a purchase of 13 more aircraft, bringing the total Airbus aircraft ordered by UAL to, I believe, 179. Delivery is in 2002.
However, to those who said UAL had a very high average fleet age, here is information straight from the horse's mouth: "At year-end 2000, United's operating fleet will consist of 604 aircraft with an average age of 10.1 years. At year-end 2001, United will have an operating fleet of 619 aircraft with an average age of 9.7 years."
I don't know what AA's or DL's fleet ages are, but I would venture to guess that UA has a younger overall fleet (especially over DL). 10 years average is pretty damn good for an airline with over 600 'crafts!