Thanks for your question Max! In the current Australian Parliament, the youngest leader of a political party is Senator Richard Di Natale. He is currently 47 years of age (DOB 6.6.1970) and leads the Australian Greens party.
Listed below are the ages of the leaders of the other parties with more than one elected representative in the Parliament:
By the way, the youngest Prime Minister was John Watson (Australian Labor Party) who was aged 37 when he became Prime Minister in 1904.
For more information about the history of parliament and its members, check out the Parliamentary Handbook.