gents.. before we all go off on different tangents...
My biggest concern .. is .. will different tread pattern really affect driving conditions.. as in different rolling conditions..
lets take as a given that tires of the same size MAY have slightly different rolling aspects, as mentioned above..
but woudlnt logic put the same tire on the front for FWD and keep the variants on the rear where both wheels roll independently what ever the conditions..??
In the case all tires are basically the same and safe, except age you want to keep the oldest tire(s) up front so they wear out as quickly as possible limiting rotations.
In your case the “old tire” hasn’t been driven on which naturally drives the antioxidants to the tires surface, extra caution run it in the rear 3-5000 miles to break it in then rotate up front and leave it.
Tires that are weather checked and dangerous you keep on the rear so if you have a blowout it has less affect on steering.
I’ve had many blowouts on the rear axle during my towing days and all were a nothing burger of firmly pull over and carefully slow.
I never was driving 90mph though or trying to do high speed maneuvers
Depending on the brand 5 years is when you start getting sidewall blowouts more often, I’ve ran them to the tire manufacturers old recommendations which used to be up to 10 years
If your car is a lighter vehicle (not like a truck or van) age is less of a concern , heavier vehicles age can be more of a problem since the sidewalls flex more and blow out more anyway.
Old tires you will want to doubly keep properly inflated, if the manual calls for something stupid like 20-28 psi don’t do that and keep it around its rating (usually 32psi+)
If the tire survives breakin the antioxidants wiill be able to get to the surface and the tire should last a normal amount of time.
Traction and wear on older “hard” tires may not be as good but it depends on what compound the brand used. Once it wears in it shouldn’t be any different than any other 5 year old tire.