unused potting soils shelf life?

yes, you are fine with it, unless it's moldy or such...

You may have to rejuventate it with some fertilizer like Osmocote. Smell it. If it smells moldy, don't use it.

Resterilise potting mix by heating it in the oven to 400 F in a roasting pan or similar, leave it for an hour or so to ensure the heat properly penetrates the mix.
If you wish to add fertiliser use a slow release fertiliser (like osmocoat), but I dont think this would be really essential. if the mix is stored in plastic bags the original nutrients will still be active.
Potting mix costs next to nothing so you may not want to go to the trouble. throw the old stuff on the compost heap or garden and buy new bags.