I found this a little interesting. Would not have been my guess of order, so I was a little surprised at some. American may have been the oldest fleet if not for the last BK filing. Alaska will get even younger when merger completed in 2019.
From Oldest to Youngest: The Average Fleet Age of the 10 Major U.S. Airlines -- The Motley Fool
From Oldest to Youngest: The Average Fleet Age of the 10 Major U.S. Airlines -- The Motley Fool