Several prominent structural failure events years back prompted the FAA and other authorities to initiate "aging aircraft" programs. When I worked for deHavilland Canada, in the Twin Otter program, I attended several FAA presentations on this. Happily, at the time the Twin Otter fell just below their radar, and was already covered by fairly rigorous life limitations on some airframe parts. Similarly, now that I work on the Basler DC-3's, I know how much goes into their rebuild, and ongoing maintenance - it's a lot! No manufacturer of an aircraft more than a half century ago envisioned the aircraft still being in service in this era, nor did they enact a continuing airworthiness program to support such operations. Further to that, aircraft manufactured for military purposes in the WW2 years were definitely not made with decades longevity in mind! It's not too much work to assure the airworthiness of a 1940's Piper Cub or Tiger Moth, but a larger multi engined aircraft is many times more effort. It nice and romantic to think of taking a flight in these very old aircraft, though the cost to assure airworthiness of the aging aircraft makes for pretty high ticket prices!