Does age of grain have an effect on efficiency?
I brewed a porter with some 2 year old pale malt and got 73% efficiency, which is pretty normal for my setup. Beer tastes pretty good, though wont be entering it in any comp.
I would think the storage would be the determining factor. If the grain is exposed to humidity it will absorb the water and weigh more. So in effect the grain would weigh more and you would be using less grain by weight.