Iv'e found that the kitchen paper idea works best, saves time & space to see if they germinate. My record is 15years out of date and most germinated (bedding aster)
I had some foil-packed T&M hollyhock seeds that were 15 tears old and had been in the garage. Result - almost 100% germination.
when i took over the garden where i am now i also inheriated loads of flower and veg seeds, all the flowers have germinated, and most of the veg has i,ll let you know more when i do the carrots raddish, when its warm enough, the only things i had a problem with was my runners beans, the broad beans were and are fine but the runners are a no show,
I've found parsnip seeds the most susceptible to age, but I'm still using tomato seeds I saved in the mid-90s. If they prove a tad stubborn, an overnight soak soon sorts them out.
I've got a 'back up' fridge in my shed which is mostly full of my seeds! I keep them in plastic containers with lids, and put in those silica sachets which seem to be ubiquitous in packaging these days.I keep on meaning to try the damp kitchen paper trick on some of the more geriatric ones, but never seem to get round to it. But this year have had very poor germination from a 5 year old packet of mixed salad leaves, so at least that's one packet to go on the compost heap.
It really depends on what plant the seed is from (and how they have been stored of course). For instance, tomato, beans, peas & lots of others will keep for sevreral years, on the other hand if you tried to grow parsnip from just last year's seed you would get jolly poor germination.
I did compile a comprehensive list of seed & how long it remains viable for....alas, it's filed in a word document, so in view of the fact I can't cut 'n paste here, I'll not be able to post it.
Further to my previous message, I've just googled this for you:
Yes, the key is how they're stored. But sometimes the apparently impossible is possible. A couple of fanatical US heirloom tom grower friends of mine have germinated tomato seeds 30+ years old.
the best thing to do is to sow and see, after all even some seeds that are in date will not germinate.
I keep ,my seed in their original little foil packets, they are all in a paper tray, which is on a top sheld in the lobby. It is draught proof and frost free. I never keep them in an airtight tin. Especially as I collect a lot of seed and it may not be quite dry enough to store. Damp seeds rot easily if airtight. I check viability of old seed by dropping in a small amount of water, those that sink are viable, those that float have had it and can be tipped out with the water.