I see this pop up on forums from time to time and then I see responses I just don’t agree with. Often people will say use the grains in 12 to 18 months, which isn’t a bad idea, but has nothing to do with the grains becoming stale. The enemies of grain are moisture and rodents. If the conditions are right then weevils can hatch and in the old days they often did. The cure in breweries was to open the doors to allow cold air in during the winter to kill the bugs. So even bug infestations are not bad for grain, but they sure make it look ooky.
So her’s my experience. Another homebrewer and myself bought all the equipment for an aging brewer and divided it up selling what we didn’t want or enough to get back the investment. What was there was some grain from who knows when and I brewed with that for years taking ribbons. Much of it was specialty grains which are not the majority of the grist, but the beer was fine. Then I started buying grains in bulk and I’ve had some of my grains for a very long time. Same deal, no loss in flavor that I have seen. I don’t enter competitions anymore, but if I did brew to style I would suggest staling is not a flaw the judges would find in my beer.
So if you have older grains, no worries, brew away. I believe the shelf life of uncrushed grain stored properly is until you decide to brew with it.