I know of no airline that has a maximum age limit for fllying; most will require a medical certificate stating ability to travel for the length of trip. I don't know whether QF will so require, but good idea to have it in any case. More pragmatically, my own parents in their 80s did fly similar trips, entirely in F with lie-flat beds, and still found it to be an ordeal (whereas in their 70s they quite enjoyed such travel). Don't underestimate how hard on the body travel of this duration can be.