As long as it's remained properly/adequately sealed and the consistency seems ok(squirt a small amount out), i'd expect it to be fine. I've been using my arctic silver for the last 6 years or so(goes a long way if you only do a system or two once a year) and it's still working just great, getting close to empty but still working as good as it always has, as far as i can tell.
As said already though, for the sake of a few quid, a new tube wouldn't hurt.