With some exceptions you need to be over 21 to hire a rental car in Australia. If you are between 21 and 25 the rental car supplier at the collection point may ask you:
At the time of vehicle collection, some rental car providers may ask Drivers over the age of 75 years to provide a medical certificate indicating they are fit and capable to drive. We suggest contacting the rental car branch prior to collecting your rental vehicle to clarify their specific requirements (if any). The final decision regarding renting to a driver over the age of 75 years rests with that rental depot location.