I worked on six aircraft by my mid-20s, if you count internships. I'm 31 now. Rich was more referring to sprawling military programs with legislative delays. You can change programs within a company, or change companies.

Also, you're kidding yourself if you think any a/c from the '50s is flying with all original parts. Like any machine, it can run indefinitely as long as you replace components. At my first company out of college, we were updating aircraft put into service in the '60s. We did new engineering to redesign parts, for example replacing an assembly of three primary components with a single CNC machined piece of aluminum. It just was not possible to fabricate the geometry with tools of that era. The new assy was simpler and stronger. Advances and upkeep like these are what keep old machines flying.