Re shelf life
, since polymer clay is only cured when exposed to a certain level of heat or UV light, theoretically it can last a reeeeaally long time before enough weak heat or light polymerize/harden it.
It will "firm up" over time though as the plasticizers sink/etc inside the raw clay, and even the oils can harden a bit with air (not technically drying, I don't think, though since no water in polymer clay).
The clay can also have some of it's oil plasticizer leached out** if it has been in direct contact with any porous materials (like cardboard, bare wood, even waxed paper...though if it stayed in a plastic bag/etc, that wouldn't happen).
You mention a "bag of Sculpey" so I assume what you have is the original "plain" Sculpey
(one of the lines of Polyform/Sculpey polymer clay). That is probably the softest line of polymer clay around so it will stay softer for longer than other lines too (though they can all be reconditioned when old, but with more effort).
Also, since your clay was manufactured that long ago it won't have the formula changes that the more recent versions have (usually a good thing for handling). But still be aware that plain Sculpey is, and always has been, the most brittle line of polymer clay after baking in any areas it's thin or projecting, will darken more easily, and has the least ability to do fine detail and avoid fingerprints/distortion/etc.
So the bottom line is that you can certainly use that clay as long as it hasn't polymerized and as long as you can re-condition it to be smooth and supple.** Check this page at my site for lots of info on conditioning polymer clay and re-conditioning even old and somewhat hard polymer clay:
Then once it's smooth and supple, use it like any other polymer clay.
(And there's more info on "Sculpey" and the other brands and lines of polymer clay too here, if you're interested:http://glassattic.com/polymer/Characteristics.htmhttp://www.craftster.org/forum/index.php?topic=366723.msg4307846#msg4307846
As for doing transfers
onto polymer clays, there are many ways to do it as well as many kinds of materials and equipment that will work. It's just a matter of getting the right combinations of those things.
So one older method was to help release the "toner" (not ink) using an alcohol of some kind for images printed with photocopiers and laser printers onto regular paper. The theory was that cheaper drinking alcohols were less refined or something so worked better ("cheap gin"). But later it was discovered that water worked just as well or better.
Most clayers who did a lot of transfers moved on to other methods though (using special papers of various kinds, etc, or using liquid polymer clay as a transfer helper--and encapsulator-- when doing direct transfers or for making intermediary decals of the transfers to apply later to polymer clay, etc).
There's a load of info on many ways that transfers can be done on polymer clay on this page of my site:http://glassattic.com/polymer/transfers.htm
...And there are even more ways/supplies that clayers have discovered since I've been able to update that page, using things like deli paper, etc, instead of transfer papers (though most all of them require toner rather than inkjet "ink" unless the inkjet ink is a permanent one).