The Australian Constitution and the Commonwealth Electoral Act 1918 (a law passed by the Australian Parliament) describe how elections should be run. The Commonwealth Electoral Act 1973 amended—changed—the 1918 Act to lower the voting age from 21 to 18. There has been recent debate about whether the voting age should be lowered further to 16, but the Australian Parliament would need to pass a bill—a proposed law—to allow this to happen.
For more information about changes to the Commonwealth Electoral Act 1918, check out the Federal Parliament History Timeline.
This fact sheet hasmore about federal elections or you could visit the Australian Electoral Commission website.