Quite a few of the seeds that came in the forgotten box my Mom dropped off are several years past the "sell by" date. Just as hybrid strains (and GMO's fall in this category)are required to be labeled, the date on those packets are also mandated. Seed companies are required to do germination testing on their seed stock. If the variety passes the allotted germination rate, the seeds are then packaged and that lovely "sell by" date is applied.
The date can vary, and more importantly, the date does not indicate the actual age of the seeds. Different species equip their seeds with a variety of built in safety mechanisms. Some are designed to withstand long storage until just the right triggers are pulled. Some won't activate unless chemical changes come about after a set amount of "chill hours".. aka exposure to cold. There are even seeds that won't sprout until exposed to high temperatures (quite clever for reforesting after a wildfire I think).
What all seeds have in common are energy reserves that allow them to hold dormancy for whatever length and provide the energy needed to break through the seed coat, send out roots and grow. When you are dealing with old seeds the mystery is... does it have enough reserves to sprout?
Many species are held for quite a few years before even being put up for the germination tests and later to be sold. As long as they pass the germination test, depending on region they are listed to be sold for the next year to year and a half. Exceptions would be hermetically sealed containers (think "life lock") some of which can list a sell by date 36 months from the date of testing (not including the month it was tested).
This is why you simply can't beat saving your own seeds. You can't accurately estimate how long purchased seeds will retain what percentage of viability over the years. Improperly placed seed racks also can do damage, something to keep in mind when you make purchases. Seed racks are placed in locations based off of grabbing a customer's attention and nothing more. Temperature fluctuations, moisture exposure and even light can significantly reduce viability of even fresh seed. Self saved seeds, properly stored in many cases can easily hold much longer than the low ball estimates of the generic seed viability storage charts.
In the case where you find an old stash.. like we seem to regularly do.. there are a few tricks to try to coax life out of them. You won't get miraculous germination rates.. but it does give you a chance that otherwise wouldn't be there.
Jump starting old seeds. When energy reserves are dwindled, they are much like a car that has been sitting for too long. You put in the key, the dash lights up, but not enough juice to turn the engine over. A way to give them a boost is to soak the seeds in water with just a touch of liquid fertilizer for a few hours and then place them in a medium to germinate.
The process of soaking the seeds help soften the seed coat as well offering nutrients that otherwise may be too depleted. Only a very tiny amount of liquid fertilizer is needed. I mean very tiny.. mainly because nitrogen can burn. Think 2 drops of organic liquid sea kelp fertilizer to 1 cup of water.
I use vermicompost tea (NOT leachate) for a couple of reasons. It is mild enough that burning is not an issue, it is what I have readily on hand, and it contains a rather balanced assortment of nutrients with the bonus of naturally occurring plant hormones. Vermicompost has it's own issues however. While the digestive tract of earthworms can clear some diseases, they also are able to spread many soil born plant pathogens.
After soaking you want to place the seeds in a medium that keeps barely damp (but not wet), relatively sterile (soil less) and protected. Some opt for using a paper towel and placing it in plastic bags. This does work well, and makes it easy to determine percentages of what has sprouted. I hate using plastic bags unless I have to, and any container with a lid works too. If you have different colored permanent markers (or sharpie pens), you can write on the towel the seed variety and the date started.
A bit of a problem with using paper towels is that the roots will go right into them and if the seeds are not spaced enough apart, and if not relocated soon after breaking the seed coat, they can be difficult to separate. With enough room you can cut or tear these individual seedlings apart from the others, making sure you do not harm the root. The root is covered in micro-fine hairs and is quite delicate. You do not need to remove the paper towel, but you do need to make sure to completely cover it with soil when replanting (otherwise exposed bits of paper towel will act as a wick drawing moisture away from the seedling).
Something that I will be experimenting with more is using a shallow lidded container and playground sand or rated sand. Rated sand is graded by particle size and is usually kiln dried. Although this takes up more room, the sand can be heated again in my oven to sterilize so it can be reused. Teasing apart the seedlings is easier is another bonus.
I just hate going through a ton of paper towels and even worse the plastic bags. You can't quite reuse the bags unless you dip them in a bleach solution as molds are a problem. Containers as well should be cleaned out when reusing. Damping off is a fungus that spreads like wildfire and wipes out seedlings in a flash.
I get a lot of satisfaction out of doing odd things like this. I have to admit my vermicompost bin is always full of unintentional experiments. Harvesting 10 pounds of potatoes from a bit of potato peelings that sprouted in the bin so far has been the biggest pleasant surprise.
I left my worms in Colorado. The ones I composted with I had collected from my yard, but bringing them out here seemed wrong as they are prolific invasives. I will start it up again with what is here.. if any are here. I have about 50 pounds of vermicompost that has been screened and frozen solid to get started with.. and that is hopefully enough.