Well, that's only according to the lyrics that I read on this one website. There might be another one that confirms that it is in fact "old."
I've sometimes wondered something similar about the "I Love Trash" song. When Oscar says, "I have here a newspaper thirteen months old," it seems that the next verse would logically be "I've wrapped fish inside it, it's smelly and COLD." But, according to the lyrics I've found and how I think I hear it being sung, he seems to be saying "I've wrapped fish inside it, it's smelly and OLD." I wonder why they would use "old" twice when "cold" would have made just as much sense. Then again, Oscar was always a non-conformist.