What airplane designs have been around the longest, from first flight through last retirement from active commercial or military service?
My guess is that the DC-3 will win, since it has been around since before WW II and is still in service, although there are enough other designs out there I'm not familiar with that I'll be interested to see where I may be wrong on that. (Russian designs come to mind as an obvious source of possibilities.)
Can anything out there beat the DC-3? What's the runner up?
If any commercial or military service will do then there are a few designs which are older.
There's a de Havilland Dragon Rapide still in commercial service doing air tours around London, that's first flight was in 1934. The Noorduyn Norseman is still used for air tours and cargo work and it was first flown a month before the DC3. The Ford Trimotor was introduced in 1925 and I think there's still one or two doing tours.
Apart from tours the DC3 is still the one I'd think of as still doing really serious service.
The Shuttleworth Collection, at Old Warden aerodrome in Great Britain, has a flyable Bleriot XI which was built in 1909 and served at a flight school into the Thirties. It is still regularly displayed in flight. You might argue that this is no commercial service, but to witness the display, you need to buy a ticket, and ticket sales help to support the collection. So I enter it as a contender here.
Having flown first in 1932, the Ju-52 is also older than the DC-3 or even the Dragon Rapide. Several are still in commercial service - you can book scheduled sight-seeing flights with the Swiss operator Ju-Air. Ju-Air's three Ju-52 served from 1939 to 1982 in the Swiss Air Force. Wikipedia has an exhaustive list of current Ju-52 operators.
The legendary Piper Cub which was used as a trainer during WWII and first flew in 1938 is still a popular plane and although not by Piper is still made in some form of variants today. I dont think its in service any more (at least as far as I know) but there are still loads of them flying.
Ju-52 started in 1931 and in 2018 - after 87 years - was still used for a commercial passenger transportation (nostalgic flights but between the two different airports like Duebendorf to Locarno all the way over the Alps and not just circular tours), by the Duebendorf based airline (Switzerland).
Unfortunately these flights were suspended after the 79 year old aircraft crashed in 2018. However the airline expects to resume the service in 2021 (source).
Older designs may serve for longer as they parts have less (or no any) strict life limits as set by the manufacturer.