The 1000 rounds of boxed 223 Remington plus other stuff I bought at auction for $2.00 recently had prices on the 20 round boxes of $5.85 per box. When do you think it was purchased?
You never did answer what type of bullets it's loaded with. If it's FMJ it might only be a couple years old if it was purchased from somewhere that priced ammo to sell. And don't let the price stickers fool you into thinking they're older. A couple of the gun stores around here still use them.
Also decent pictures of one of the boxes might help to narrow down a date.