It's less about the freshness of the grains and more about the degradation of the supplemented vitamins, particularly A. When a bag of any feed is stored for extended periods those supplements break down and are lost, and you are left feeding a product that you think is 100% complete but sadly, is not.
I fully understand,but, my point is this 'degradation' happens with the raw ingredients as well it's not just happening on the prepared food and the chances of being short nutrients when you mix your own is likely higher than the marginally lose in a commericial feed unless you regularly send your mix out for analysis...