That's disheartening to hear, the Walmart tire shop refused to fix your 6 year old flat tire? Was there bulging, cracks, or cords exposed? They refused to fix it only due to age?
I had the same experience at Discount Tire in 2022 on a tire that was purchased at their store. The manager refused to patch a small nail puncture in the middle of the tread because he said the tire was just over 6 years old. When I referenced the policy on the DT website that says they will refuse to service tires over 10 years old, he stated that his discretionary threshold was lower because most tire manufacturers "recommend" replacement at 6 years.
I drove home and repaired it myself with a Safety Seal plug.