I had a bottle of Green Valley that smelled like stale berries on top, and the inside of a football player's cleat in the drydown. No doubt in my mind with this one - give it enough time, and it will turn rancid.
Green Irish Tweed - I owned a bottle that had technically turned, but it didn't harm the quality of the scent. The top notes were very spiky and had a sour edge to them, but somehow this did not effect how good it smelled. The drydown was just fine, exceptionally smooth. That bottle was ten years old.
My current GIT is a year old, and it smells better now than it did when I bought it a year ago.
I think taking care of the scent prevents the aging process from damaging the fragrance. Leaving it in the sun, on the back step, or in the ice box may change the scent and destroy it, as with the GV.