The KC-10's are almost as old as the KC-135's. How many 707's and KC-10's
are still active in commercial service? I think Fedex dumped all of their
10's a few years ago.

Still, flying in a 135 is a nice quiet ride. However, the airframes are
aging, and it is becoming more and more expensive to maintain them, to the
point where it may no longer be cost effective.