It's difficult trying to get my head around this.
Why do you think the scenario would be unlikely?
I think I'll butt in here. I'm also quite young, but maybe we'll think on the same level lol
The AUD/USD will most likely never "fail", as a failure of a currency basically means it is worthless, and unable to be used in exchange of goods and services. How do you make a currency worthless? As others have stated, take a look at Zimbabwe. A true basket case, they print so much money that it's barely worth more than the paper it's printed on (hyper-inflation). That currency has failed.
I find it highly unlikely that the printing presses will ever dish out such a vast amount of dough in Western countries simply because we know what will happen - utter destruction of our economy.
What some commentators are concerned about is deflation, or an exteme case, depression. In this case, it's almost the opposite of what you're looking for. If it were ever to happen, the AUD would not fail (as in be worthless), but actually be worth a lot more!
So if you're scared now, thinking a depression will come, horde cash or precious metals (gold/silver). Unforuntately, that very thinking spread across the whole population could cause a depression in itself.