Hi and welcome to FT!

No special clearance (medical certificate) is required due to her age alone.

A medical clearance may be required if you, your doctor, or the airline isn’t sure of your mother’s fitness to fly. Full details can be found here: https://www.qantas.com/au/en/travel-...ss-to-fly.html

If your mother is able to walk to/from the plane, and doesn’t have any obvious signs of illness or a reason for the checkin staff to suspect the flight might be a danger to her health, there shouldn’t be any need for a special clearance.