We guarantee germination of our seed kits for one year after purchase from any authorized seller. 
Generally, the seeds in a seed kit will remain viable for two years from the date of assembly if stored in a cool, dry location. As seeds get older, their germination rate falls and the average number of days to germination will rise - so if you have had a seed kit for a couple of years, give it a little more time to germinate.  

Critical to seed life is proper storage.  Store seed kits where it's cool and dry, for example inside a baggie in the refrigerator, and they will keep longer than at room temperature.  Don't store them where it's warm and humid;  they may want to start germinating inside the seed kit box and then be spent by the time you plant them.

The nutrients have a long shelf life and need no special care.