I bought a kit over the summer from Northern Brewer. This kit comes with the malt extract and the grains (for color mainly). I assume the grains would be stale, so I could probably buy some new grain, but I guess the extract would stay good for a long time, right. Should I just throw it out, or is it still good?
If everything has been sealed tightly, it should all be good still. The grain is probably fine, check that it's dry and still hard before using it, and inspect carefully for weevils or other small bugs.
Dry extract is good for at least 6 months if sealed and dry. Sift it and see how clumpy it's gotten- if it's been good and dry, it won't have many clumps, and you can still use it.
Liquid extract is fine for at least a year if it's sealed. Check the top of it for mold.
There are many schools of thought on hops going bad- personally, I'd buy new ones. Also buy new yeast, or at least get it started a couple days ahead of time and make sure it's nice and alive.
If everything is reasonably cool and dry, no problem, but it would be a better if the hops and yeast had been kept in a refrigerator. You might have lost a little hop flavor, or the ferment might take a bit longer
Remember that barley and hops are only harvested in one season - does AB stop brewing every spring while waiting for the next harvest?
A few bugs on the grain isn't so bad (been there, brewed that) but mold is bad news, will actually screw up your beer (apparently some of the mold's enzymes will survive the boil and keep digesting your beer).
Dry yeast is supposed to be OK for 1 year at room temp, but it will stay viable for quite long time if sealed. That stuff is hard to kill completely.
As far as the extracts go, I've never heard of them going bad, as long as dry is dry and liquid is not diluted.
Best of luck!