I've had seeds germinate from packets which are 4 or 5 years out of date - not as reliable as fresh stuff, but following the advice of D.G Hessayon I usually plant two bean or pea seeds where I want one plant - usually I got two plants and had to choose the stronger one anyway. Obviously seed merchants would like to sell you new seeds, with a guaranteed high germination rate, but as I've not yet managed to plant enough of one seed type to use up a packet in just one or two years, I'm often using out-of-date seeds.
Does anybody know if there's an increased risk of disease or weak plant, if you use an older seed? Or, assuming it does actually germinate, can you expect the same growth as with fresh seed?