...the first KC-135A was purchasedby the U.S. Air Force in 1954; I know the B-52 "BUFF" went through several models, the earlier ones (A, B, D...?) having been retired to the "boneyard." The current B-52s are G and H models, I think.....??? and so I doubt they are as old as the KC-135s that are currently flying (the last KC-135 ride I took was on an aircraft with a 1957 tail number....!). I am not sure; never worked on any bomber aircraft; all of my Air Force service was on fighters and the A-7D series... the Boeing 767 series might very well be under consideration for the Air Force's next-generation tanker....
You make a valid point. However, I was referring the introduction of the "airframe", not how many revs and updates it's gone through.