To elaborate on some of the things that were mentioned, crushed grain stored dry and cool will stay good for several months, but its quality will degrade with time as it absorbs moisture and oxygen. I prefer crushing my grain immediately before brewing to lock in as many of the more volatile aromatics into the mash (or steep) as possible. When this is not feasible, I recommend vacuum sealing at least the specialty malts in your recipes to slow down staling. An inexpensive vacuum sealer is a very good investment for a homebrewer.
You can still make very good beer with old crushed grains though. I have done it, and the difference is fairly subtle in most styles. But as you improve your skills and try to make the best possible beer you can, fresh or vacuum sealed and frozen ingredients are the only way to go.
