Baron, for an answer, go to the 'search' function and do a search for 'cadets' or somthing like that. You'll find the identical question and a bunch of answers (whether you think they are valid or not is up to you to work out) but this one has been done to death for a long time.
Short version. Cadets from early '90s was when a pilot shortage was looming. QF signed contract just before the '89 dispute and so proceeded with half the course. 90ish cadets from about 97 onward were fulfilling the residual numbers left over from that early course. Since then, dunno.
However, to present an alternative view, how about this. Perhaps it is a similar principle to the RAAF. Get them in early. Inculcate them with the culture that you want, train them the way that you want with the attitudes that you want. Get them to hang around for long periods of time due to loyalty to the company that gave them a break and ultimately have a 'pool' of people joining at a young age that should provide an effective and experienenced pool of pilots that may provide a few good'uns for possible management slots.
Interesting though that although the principles are similar to the RAAF, I've yet to see a thread raging against the ADFs selection of what is essentially a 'cadet' for their flight training. I wonder why that is!
(Yes, I'm an ex cadet. Qantas 8 graduating AAC Sep '92. F/O for 7 1/2 years. First bloke off our course starts command training with AO in Jun. I wonder if the sky will fall then!
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Cadets are highly motivated, capable pilots. Out of the few dozen that I've flown with I've come across one that I would put in that category and he had toned down by the next time I flew with him. As to the myth that they are 'rich', a few that I know pretty well had their parents extend the mortgages on the houses to pay for the cadetship and the cadets themselves are STILL paying it off in return.