I have bags for my vacuum that are about 10 years old (bought 'em then misplaced them and only found them recently)
There is no evidence of degradation in their quality as compared to newer bags.
I'm sort of in the same boat as you. When I heard that my Eureka model was being discontinued, I stocked up on a bunch of bags but there is no "best before date" stamped on them. Yup, these bags on pretty close to being 10 years old as well. My Eureka is 37 years old and still ticking.
I'll look up the model and give the internet a try. This is one of the problems with old equipment. A friend of mine has a 1969 Firebird (beautiful car!) but he has a huge problem when he needs to replace a part because they are really hard to find. Sometimes it is possible to get a part from a scrapyard but it is hit or miss and often a miss.