At a friend's house, we found out his mother used to smoke pot in her old hippie days. In the attic, we found ten seeds that were apparently over 15 years old. (after asking his mum) They weren't even correctly stored, but after germination five of them sprouted, we put them in the garden, laughing a bit, thinking that of course none would survive. Little did we know that two of them were indeed female, and we had a little harvest.
But the real "expiration date" of seeds, well that's for the expert to share with us.
My advice: TRY 'EM! (but watch out when transporting to outdoor environment, they might need a little more care)