As a rider of various subway/metro/underground systems I was curious where would I find the oldest fleet still in operation.
NYC subway C line has cars built in the mid 60s. I was wondering if there is anything older than that?
If Wikipedia is up-to-date, then Buenos Aires has a good claim to having the oldest subway coaches in commercial service in the world, built between 1913 and 1919. They have been running on line A since its inception. These trains were being phased out as of 2010; I'll let someone with a better grasp of Spanish determine whether they are still running now.
I thought some impoverished East European cities had old stock, but they all seem to have renewed anything pre-1970.
The London Underground ran A60 and A62 for about 50 years, since the early 1960s. These trains were due to be phased out in the early 2010s; the TfL plans called for A stock lasting until 2015, but the replacement seems to have been expedited. The Metropolitan line might still be running some. In the Tube, the 1959 stock is no longer doing passenger service but is still used for deicing in winter.
If you're willing to accept streetcars, then Hong Kong is still running rebuilt cars from the 1950s (older cars from the 1930s have been scrapped).
The Isle of Wight line (in England) still has 1938 ex-London Underground rolling stock in active service.
The Schwebebahn suspended monorail in Wuppertal, Germany, not far from Cologne, still trots out Emperor Wilhelm's Kaiserwagen, built in 1900, for special occasions and tourist charters. It's not in daily service though.