I stand corrected. My excuse could be that I meant direct combat aircraft but I cannot tell a lie, I was just wrong. KC 135 is a derivative of the 1950s Boeing 707 and yes it is almost but not quite as old as I am.
I flew in a C5 when I was in the Navy back in 1985...
Incredible longevity for that aircraft. It entered service my senior year of college and I am now 72. I know there have been some upgrades. It is still our fastest fighter plane.
It has to be a solid number two behind the B-52 for longevity.