You can get a student pilot licence at the age of 16. You would also have to sit for a Basic Aeronautical Knowledge [BAK] exam that is administered by your chosen flying school. Providing you have passed the BAK exam and had it certified in your log book, you could sit the CASA PPL exam at tdhe age of 16.

Note that after September this year, when new regulations will apply, that will be lowered to 15. But you will still have to have passed your flying school's BAK exam before you sit the CASA PPL exam.