About a year ago I bought 9 bags of grain, and stored them in the bags in plastic tubs (not airtight) in the crawlspace, where the temp is pretty constant at about 60 or so. Beer I made recently was just as good as beer made a year ago. The grain tastes and smells good.
When I open a bag I usually put the rest in plastic buckets with tight lids.
I think its possible that the grain could be over a year old before it gets malted. It may have spent a long time in a grain elevator or silo. I would think that the shelf life is less once its malted, but I don't think that even malted grain is particularly fragile.