The root problem is that organic materials, like the rubber compound of tires, degrade with exposure to UV light, temperature extremes, and oxygen. The polymers break down and the material's properties change.
In tires, flexibility will change, which will affect the grip and performance. Evidence of this can show up as cracking.
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A large body of scientific evidence supports that most tires should be replaced six years from the date they are manufactured. This six-year expiration date begins from the day the tire was manufactured at the plant—not the date it was sold to a consumer or the date that it was installed on a...
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It's also the reason why vehicle fluids need to be changed. Aside from the fluid simply getting dirty, the polymers break down and diminish in the properties they were designed for like lubrication, anti-corrosive properties, viscosity, etc.