I came across C-47 ( Dakotas) still flying in Canada ( Calgary) in the 80's. IMO what is more important than age of an aircraft is its upkeep. I have flown in F-27 dozen of times during the 70's working for Esso. It was the only aircraft availble to hop from Lahore to DI Khan and then to Faisalabad ( Lyallpur) and back to Lahore. I found it to a be a sturdy and good aircraft. There is no reason why if properly cared for, F-27 wont be able to fly for say another 10 years. Besides, saftey standard required for a Civilian Passenger aircraft are a lot tougher than for military missions.
I remember a Naval Commander informing me that F-27 was able to look 70 miles ahead and proved very useful for recce and liaison work in the PN during 1971. Additionally, it is also possible that after P-3C's take over MRP role, Fokkers are transfered to the Coast Guard. To the best of my knowledge US Customs and Drug Enforcement still use Fokkers for patrol along the Mexican border.