The F/A-18E/F Super Hornet, if you haven't noticed...
I believe the Hornet first flew in 1978 (which would make them 30 this year), but it really gets down to how and when each Block was delivered and how many were in each Block before you can talk about how long they may be around. Super Hornets are going to be around a very long time - I'll probably die before they do, at this point in the game.
I think Canada was the first FMS Customer, and their As would be the oldest serving in the Fleet next to the USMC and USN As, unless I've got that backwards with Spain - and both the CAF and RAAF have just upgraded to the A+(+) config and will keep them going for some time longer.
Now that the RAAF have bought the Super Hornet and opened things up for sales, other countries are looking at it as well, but it's a choice between the Super Hornet, Typhoon, Grippen, and waiting for JSF in the NATO marketplace at the moment...which is why I'm very skeptical about JSF.