Tire age

The only way to " thoroughly inspect a tire" is to send it to a lab where they can use their test equipment on it and intentionally destroy the tire. then they do an "autopsy" on the various components testing the different materials and chemicals used in the tires manufacture.
Only Superman had X-ray vision.
Tires have very little "rubber" in them. Mostly petroleum derived synthetic compounds.
Want to see evidence of old tires with new tread that fail?
Watch the freeways during the summer and you will see delaminated tread all over the sides of the road.
Note : the Motor homes, boat trailers, travel trailers etc... that sit all year and get taken out for summer vacation.
Low air pressure is like running low coolant in your radiator. Heat builds up to the point of tire/engine failure.
I work both Firestone recalls. The first one was at the time when radial tires were a new concept to the American market.
Most manufacturers suffered tread separation problems and a lot of testing was being done using all types of belt and carcass materials.
The strongest being Nylon. But Nylon gets stiff when cold and would "thump" in the morning until the tires warmed up. So it got relegated to commercial tire use since comfort was secondary to strength and durability.
Polyester, rayon are the most common carcass materials. Each have their advantages.

The second recall in 2001 was more of a family/business issue. The President of Ford at that time was Jacques Nasser. When Ford was being sued for roll over deaths on their Ford Explorers, which were equipped with Firestone made tires that were spec'd by Ford for the Explorer Jacques blamed Firestone and he threw them under the bus on tv. eventually he was fires and Bill Ford jr. assumed the presidency of Ford. By the way he was Bill Ford Sr and Martha Firestone's son. Henry Ford and Harvey Firestones great grandson.
Most don't know that Ford reimbursed Firestone for the recall. and the Explorer was completely redesigned as well as the air pressure requirement was raised.
We found that checking the psi of the Explorers coming i
n for replacement tires averaged only 18 psi in their tires. People just don't check their tires air pressure.
Heat is always the enemy. Low inflation, low flexibility ( Hard rubber) generate friction= heat.
Heat causes the tires compounds to delaminate/disintegrate causing catastrophic failure/blowout of the tire.
High speeds generate high centrifugal forces on the tread/belts.
Tires were not designed to just support the weight of the car. A balloon can do that.
They are designed to provide traction and stability for your car.
Keep your old tires for rollers around the shop or for display purposes, but don't risk your self or others safety to try to stretch the time you think your tires are safe.
Body work/damage isn't cheap either.
Be safe,
Check your air pressure monthly. and replace your driving tires whenever the tread is less than 4/32 and/or over 6-7 years old. Don't gamble on a few extra dollars saved.
You and yours are hopefully worth more than that.