How long does gunpowder last?
 

We have about 5 containers of gunpowder downstairs, its all about 15 years old. used to reload centerfire rounds. Is it still good? we havent used since it was bought

Should be good as long as it never got damp. You will want to try your loads at the range and confirm velocity and accuracy though. It may just be cheaper to start from scratch again, bite the bullet and buy some new stuff.

I've used both powder and primers that was over 30 years old and it chronographed the same as a new lot I tried.
If it has been stored dry and cool, it will be okay.
When you open it, if you get an acrid smell and some reddish tinge around the lid, it will need chronographing to be sure it is still decent.
if however, you get that "nice ether smell", she's goodtogo!:)
Cat