Last week I used a tube that had been opened and then sealed with duct
tape. It was originally used a couple of years ago-- and it worked
just fine. I'd used only an inch or less of it, so there wasn't much
air in the tube. Presumably yours, unopened, should not have hardened.

By the way, mine wasn't water-based, or whatever the easily-washable
stuff is. It was the silicone kind that's really annoying to wash off
your hands.

Comedian Shaun Eli
http://www.BrainChampagne.com
Brain Champagne: Clever Comedy for the Smarter Audience (sm)