Hi ncdaisies,
I read on one website that zinnia seeds remain viable up to 7 years. It probably depends on the conditions that they are stored under. I have used seed that was 2-3 years old and had good germination rates. I even planted some celosia seed this year that I found hidden on a shelf that had been there for 10 years. I was surprised that probably 70% of them germinated. If you go over to the Annuals forum you will find a lot of useful information from threads by maineman on breeding zinnias.
BTG