In Australia and most countries, you must be aged 21 or over to rent a car and in any case have held a driving license for at least one year.
Young driver fees :
Lucky enough to be under 26? If so, a small Young Driver Surcharge is applicable in Australia when you are hiring the vehicle in.
This information can be found in the Specific Terms
The age requirements may vary according to the country and to the vehicle category considered. For a better understanding related to your country of rental, you can find information in the Specific Terms and Conditions per country.