Don't mean to be picky, every airline has its +'s and -'s.
Currently Jetstar's average fleet age is about 3.5 yrs overall (As of February 2008, Jetstar's average fleet age is 2.7 years) and the Int'l route is run by A330-200s which are a little older than the A320s but only a couple of years. The 717s they were flying with at first were old from memory but they phased them out, a fleet on modern, new aircraft means less maintenance costs as aging aircraft need more regular checks.
The average age of a Qantas plane is over 10yrs old, and the average age of a Qantas 747 is over 15yrs old.
On a useless note
I do agree about customer service and other factors on cheap airlines being a bit 'crappier', and I got hit for over $160 for around 15kg too much baggage Melb-Brisbane recently on Virgin so I'm stil not talking nice about them.
I fly with whoever has the cheapest usuallly, paid full cost for Singapore Airlines to UK and back last year and didnt get even enough frequent flyer points on the Air NZ partner scheme to fly Wellington to Auckland, but being a longer flight the added features made it all the more enjoyable.