Not sure how old the Harpoon stocks are, I'm sure I read something on the subject but can't remember where now.
Probably from early 1980's, some may be newer, Hellenic Navy also has such old Harpoon stock, purchased with the Elli Class frigates in 1981-82. Harpoon is turbojet powered, so I believe that the problem is the warhead and electronics-if something in there has specific life. Fuel also could be a problem, not sure about what fuel the Teledyne J402 turbojet burns-that is the engine that powers Harpoon missile, at least Block I round. In Exocet MM38 anf MM40 Block 1/2, only problem with age is the rocket engines, which need replacement every 10 or 15 years (not remember what I have read about), warhead is much more long lived if maintained and stored properly. May Harpoon's booster needs also replacement or rebuilt after its life ends.