Hello Mate
I have just finished flight screening. I lived for 26 years thinking that it was next to impossible to get a start but I tried and it is possible. Nothing can prepare you for the final stages but working on mental maths and showing you have the desire to fly helps. Leadership potential is a major factor too.
On my FSP course there was everyone from 20 yr old Aero Eng students to F111 Nav's to an old banker like me. You probably won't guess who scored highest.
Medical is a big barrier for most. At your age it will be easier to try for ADFA than DEO.
Regards