While some seeds may last for a long time (like tomatoes), others start to lose their oomph very quickly. I have found that with onions it’s best to purchase new seeds every other year. In general, fresh seed will have better germination rates than older seed. Tomato seeds seem to be the exception, I have great germination with old tomato seed. Beets do better if they’re only one or two years old. Onions need to be fresh.