I want to make pastrami after finding a recipe here. Can i use Curing salt that has been hiding in the back of my cupboard for 3 - 5 years
I don't see why not. Salt is one of the most inert food ingredients on the planet since the dawn of humanity.
If you have any doubts, your wisest course of action would be to contact the ingredient manufacturer for their advice.
Best of luck.
Yes, that should be fine. My curing salt is about that old and I have had no issue making corned beef with it.