I'm using seed that is three years old, a black tomato heritage seed, and the plants are two foot tall already. As long as they are kept in a cool dry dark place, mine are in a chocolates tin, they should be fine. I've still got some for next year too.
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Tomato seeds are one of the ones that seem to remain viable for almost ever if stored in cool dry conditions.
next year.
I’ve not long finished a packet my dad had started dated 1995.