I have drained "near-new coolant" from rusty systems, let the crud settle down, and put it right back in another system.
Coolant never "wears out" - Several engine manufacturers suggest doing an analysis to see which additives are depleted (similar to a UOA) and the additives can be bought separately in small bottles to add to the coolant to replenish it.
"The evidence demands a verdict". (Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."
Valvoline lists shelf life for their Zerex brand antifreeze products on their web site, in the PI sheets. I use those as my go-by, it varies from 2 years to 5 years depending on specific antifreeze type.
You could call or email Valvoline to get their viewpoint on what happens when antifreeze shelfife is exceeded.
"No matter how paranoid you are you're not paranoid enough. Tell the truth. Reach as many people as you can with it. That's your weapon." - Susanne Modeski, aka "Holly" to The Lone Gunmen
About 6 years ago I put traditional green Zerex made in 1998 (from an unopened container in my garage) into a newly purchased (used) 2008 Hyundai Elantra. When I changed the timing belt 2 years later the water pump looked brand new. I don't think unopened coolant deteriorates over time enough to worry about.
PS: None of the coolant had settled out in the container despite sitting all of those years....
Yes it degrades and can go bad. The inhibitors do break down and don't protect metal as well as fresh AF. Also Coolant becomes Acidic and corrodes metal. To stay on top of the game...you can always buy test strips and check your fluid every year.