"My Ford truck (diesel) is a 2000 year model and has never had the antifreeze changed or even added to for that matter. Thats 15 years. It doesnt look worn to me. I think I'll keep running it awhile longer."
The hassle with old coolant (over 3 years) is it can turn into a mild ACID..
That is why most AF mfg ask you to change it , newer engines have lots of aluminum , that dissolves easily.
Any drug store or truck stop will sell PH strips to see .