Dried sea vegetables last a very long time when they’re properly stored in airtight containers, out of direct sunlight, and not exposed to long stretches of hot temperatures.  Nutrients such as minerals, iodine, and fiber are likely to remain wholly intact in dried sea vegetables for many years, although no one has ever shown this scientifically. The flavor and texture are also likely to be unaffected. However, it’s possible that some of the plant’s vitamins and lipids may deteriorate after many years’ storage.
Generally speaking, we label our sea vegetables with a “best before” date of 3-5 years from harvest.  This date can be found on a white "BEST BEFORE" sticker adhered to the package.  However, just because your sea vegetables are past this date (or the sticker is missing) doesn’t mean they’re bad and should be thrown away.  We’ve had customers report that dried kelp purchased over 10 years ago was still as good as the day they bought it.  Ultimately, you’re the judge.