Having read Veggie's thread about 2,000 year old dates (I think she placed them in a box and forgot about them) and I am sure there is a reference to an exhibition strain of onions coming from a packet of seeds that was in a jacket pocket for 7 years, made me wonder how many gardeners have had good results from seeds which have been well past their best by date, personally I haven't a clue as I don't keep records of seed sowing etc. but perhaps you have, or have heard of good results from old seed, if so please tell.
Tomato and bean seeds are quite vigorous, well after their sow by dates.
According to Lawrence D. Hills, pumpkin, courgette and cucumber seeds should keep for 6 or 7 years with old seed reputed to have a higher proportion of female flowers.
I had problems with DFB’s not growing from seeds that were about 3-4 year old, threw them out and bought fresh seed which grew late in the season just gone.
I have successfully sowed peas that were easily 4-5 year old, just steep tnem in water overnight put them sitting on some damp potting compost and watch for the little tails, success rate about 80% sprouting all sprouted plants grew.