Answer: To some extent the viability of the seeds depends on what they are and to some extent how old they are and to some extent how they have been stored. Storage at a steady cool temperature and under dry conditions (such as in the refrigerator in a closed airtight container) for example is better than sitting on a shelf out in the shed. Some seeds are long lived to about five years or so (tomatoes, peppers) and others such as corn and lettuce may not last two years. If you are willing to experiment a little it might be fun to see if any of the 20 year old seeds will germinate, but I wouldn't count on them. You can test the seeds by placing a sample on a damp paper towel and wrapping that in plastic to keep it damp. Set it in a warm bright location out of direct sun and see if any swell and germinate. Use the results as an indicator for the remaining seeds.
Tomato plants should be conditioned or acclimated gradually to the out of doors over the period of a week or so. Once all danger of frost has passed and nights are in the mid forties, the soil should be warm enough to plant them out successfully.
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