These chemicals should last for many years, probably decades, without significant degradation. It is unlikely that the age of your chemicals has anything to do with what you are experiencing. However, sulfite will in fact off-gas over time. If you don't bottle right away, if fermentation is still happening, it will be reduced over time. Sorbate is more stable.
Refermentation can happen from time to time. There are many reasons, a few of which I'll cover here to give you some things to think about.
Sorbate and sulfite don't kill yeast or prevent healthy yeast from continuing to ferment if there are sugars present that they still want to eat. Sorbate neuters the yeast so they can no longer divide and produce more yeast cells. Sulfite keeps bacteria and wild yeasts at bay. The two substances can work together to stabilize a fully fermented cider. However, if the yeast is still happy and healthy and hungry, and sugars still present, then refermentation can still happen. It's just less likely to become problematic to where you would have exploding bottles, etc.
If you want, if you haven't bottled yet (sounds like not), you can try adding another dose to see if that calms the yeast down again, which may or may not work successfully. Alternatively, you can leave the cider alone for another month or two until there are zero signs of fermentation, then stabilize once again, with better odds of success.
Bottom line: Ideally, you want your yeast to be really tired and basically wishing they were dead before you add sorbate and sulfite and any backsweetener. This will improve odds of success in stabilizing. You won't be very successful in stopping an active fermentation with sorbate and sulfite. It has to already be pretty much finished. If fermentation is happening in your cider right now... you might be best off leaving it alone for a bit longer before taking any further actions.
Cheers and good luck to you.
