I recently used a nearly 4-month old pack that seemed to behave pretty normally, and a different 6-month old pack that did nothing in my starter. So somewhere between 4 and 6 months may be your limit. Note that this is in the context of pitching the pack into a 1-liter starter and letting it go overnight with occasional shaking.
What is more important than how old it is, is how you treat the yeast. It would need to have been refrigerated and never have gotten frozen or been too hot.
You will need to make a starter. And the size of the starter depends on how old the yeast is and what the gravity of the recipe is.
Mrmalty.com has a calculator that will give you the proper size for your starter taking into account the production date of the yeast.