I was between the ages of 11-13 when I got arrested for vandalism and had to do some community service. Then I got arrested for trespassing around the same age and I'm pretty sure they just let me off for that because nothing came of it.
When I was joining the military a few years ago, they tried to pull my criminal record from the police station that should have it, and nothing came back. What do I do? What would you do?
You need to be very clear of the meaning of the word "conviction". A youthful encounter with the system is a far cry from an adult conviction.
It is quite possible you were diverted into a program where they filed charges and put them on the shelf pending you doing some community service or other reparations, and then dropped the charges. It's easier! It avoids a bunch of hard problems relating to getting a minor to consent to a plea bargain. Establishing that an adult plea bargain is valid and consensual, so it can't be appealed later, is a hard problem, actually. Harder with a child.
The exact workings of this are beyond our expertise in travel.se, and you either need to go to law.se and give a lot of personal information, or have a local attorney investigate. I would recommend the latter.
I would not give a response to Australia until you have, because if you incorrectly admit a conviction that did not happen, you will never be able to un-ring that bell.
The official PDF help for the Australian ETA shows the question. And states :
If the applicant has been convicted of an offense in any country (including any conviction which has now been removed from official records) this mean that the applicant has a criminal conviction
If the applicant has a criminal conviction in any country, they should apply for a Visitor (subclass 600) visa
You can try applying for the ETA, but it is extremely likely that you aren't eligible for an ETA now and must apply for a full Visitor (subclass 600) visa
