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Although Dexcool is supposed to be good for 5 years, I never let mine go more than 2 or 3 years.
Especially if you are not going to completely drain and/or flush the system. I usually just drain what I can (about 8 quarts) and refill with a 50/50 mixture of coolant and distilled water instead of flushing all the coolant out of the block.
.....but.....if it looks very clean and doesn't have any "floaters" in it I would not change it. You are very low-mileage and the fluid is supposed to last 150K.
Change ALL of it now. The coolant get acidic as it gets older. This will damage the seal and bearings on the pump. It is cheaper and eaiser to replace the coolant than it is to replace the water pump. Use distilled water and NOT tap water to FLUSH the system first. Then refill with 50/50 distilled/coolant mix. Cheap water pump insurance.