My son and I recently bought new trucks with identical OEM tires. He's upgrading his tires. How long can I store his pull offs without affecting safety of the tires? I don't drive much so expect 4 years of storage followed by 4 years of use. Would be stored in an unconditioned shop.

Factory tires are kind of crappy, best use would be for both of you to wear out the factory set before replacing

But if he's determined to replace, the general rule of thumb is 7 year life. I would expect that to be longer for tires that are stored out of the sun. Maybe even cover them to protect as much as possible

Second consideration is use. I'd avoid old tires for high speeds or crowded freeways. If you're puttering around the farm, or other relatively low speed applications with a skilled driver, I wouldn't worry as much.

Seems appropriate to say "your mileage may vary"

Just read a deal in Grassroots Motorsports on this very topic - best to store them in plastic trash bags sealed to keep the oils from evaporating as quickly. And best to store in an air-conditioned space as the heat keeps the vulcanization process moving more quickly. Obviously avoid UV exposure, but also avoid ozone exposure, so not near a gas furnace, water heater, or electric motors. And don't store them next to chemicals.

Since you already said they will be in an non air-conditioned shop, try to adhere to the other conditions as best you can and I'll bet you'll be fine within the timeline that you will blow through them anyway since they are OEM tires good for maybe 30k miles.

I just bought new tires for our "extra" vehicle. We kept our 2002 Sequoia for a camping/Home Depot runs etc when my wife got an MDX. The Sequoia sits most of the time but I have been using it a lot recently. I knew the tires were old and I started to get nervous about how old they were. The date code was on the inside of the tire so when I finally got under there and saw that they were 12 years old I knew I was about to spend some cash. The tires had 10's of thousands of miles of tread left, but they were just too old to feel comfortable driving on the highways. $600+ on tires that likely won't see 10,000 miles I feel much better about driving that vehicle.
The Sequoia has 214k miles. The '15 MDX now has 210k miles. We like to keep our vehicles for a long time so we buy vehicles that last a long time…Japanese!