We have actually had a guy come in every few months or so and have us transfer some old ones he had from the 70's. Now I can't verify that is actually when they are from for sure, but he would have no reason to make that up. Plus the "content" of the transfers and the overall appearance wouldn't surprise me at all if they were from the 70's.
I have ones that we printed that are a little over 6 years old, and they work just fine.