Coolant. Can coolant over 5 yrs and 100k miles protect your car in 20 degree weather?

Yes, easily. If it is five years old it is probably HOAT, with a change interval of ten years / 100K miles so it is about due, but that is for corrosion resistance. For freeze protection it will easily make -20F.

Unless it's been diluted in the 5 years, protects against freezing as well as when new. But the anti-rust/corrosion additives, water pump lubricant and other properties degrade with time. Follow replacement info in your Owners Manual.

Go to a auto parts and get a anti freeze tester, and see what it says. that anti freeze really needs to be changed thats in it now. thats way to long to leave it in there.

It should. You can get a simple cheap device at the auto parts store to check it. You siphon a bit of the antifreeze into a clear chamber that has either different colored balls or a floating indicator. It will then tell you the specific gravity of the antifreeze and you read a table to see what temperatures it will work.

I don't think I would trust it. I use a quality coolant system called Nal-Cool, and every 3 years, a complete flush and coolant replacement. Never had overheating or freeze problems down to -10*.

(radiator and cap looked like new after 6 years.)

Coolant should be checked yearly if temps go below freezing.