HR noted that 1Deng was losing and was worried that 1Deng would be defeated by Golden Wheel. They had barely started fighting.
QQR fought Golden Wheel over 1000 stances and over a night, made it back to find 1Deng, traveled to find Yinggu before dying.
I don't believe in the old age stamina thing because logically, with time, you can cultivate more inner energy and that should provide the fighter with more stamina. There is no evidence that suggest older fighters are at a disadvantage.
Also, I don't know how far apart the ages of QQR and 1Deng are.