Do iron-ons expire

It would depend on the quality of transfer paper the iron-on had been produced on. Like everything, the paper is subject to corruption and age and the iron-on may have cracked with age. This will become apparent when you try to apply it as it will either fragment or go on "crumbly" or look a bit blurred. We're talking a very long time here. Most iron-ons, if they have been stored well, not exposed to excess light or moisture should apply fine with the application of heat. If worse comes to worse and you are unsure you can always sacrifice one and test it on a rag, turn the rag inside out once the image has set, wash it, and see how it comes out.