Military aircraft can have notoriously short lifespans, especially during periods of technological ferment. The most elite aircraft of World War I could become obsolete in a matter of months. Things weren’t much different in World War II. And at the dawn of the jet age, entire fleets of aircraft became passé as technologies matured. The advanced fighters that fought in the skies over Korea became junk just a few years later.
But a few designs stand the test of the time. The B-52 Stratofortress first flew in 1952, yet remains in service today. New C-130s continue to roll off the production line, based on a design that became operational in 1954.
But those are bombers and transport aircraft; they don’t fight one another. Fighters face a special problem of longevity, because they must compete directly with newer models. Thus, very few fighters have had long lifespans, either in production or in service.
The MiG-21 “Fishbed” is an exception.