Eighty year old submarines are, generally, not the first thing you think of when asked 'oldest warships in service'. You probably think of Connie or Victory first, though they do stretch the definition of 'in service' a bit.

However, the Taiwanese Navy (Republic of China Navy) does still operate two WW2era submarines. The exUSS Cutlass and USS Tusk. These are pushing eighty, and are still in limited service. They probably won't be for much longer.

But for now, these remain the oldest submarines in service by several decades.

Further Reading:
https://www.history.navy.mil/research...
https://www.history.navy.mil/research...
https://www.taipeitimes.com/News/edit...