Hi Carlitos,
I don't think age has much to do with autumn colour.
As has already been stated, the main factors that influence colour are genetics (choose trees for autumn colour in autumn, some individuals have better colour than others), and environmental conditions - cold nights and warm, sunny days give maximum colour.
Here's a photo of the same Japanese maple as above a few years ago when conditions were even better.
AP 3 2009 05.JPG
In Melbourne you tend to have more overcast days and no really cold nights so trees there will rarely display their best autumn colours (unless you resort to the beer fridge as Mojo suggests???)
You could try moving to North East Vic or stay in Melb and just enjoy your trees as they are.
For those who have a bit of time a trip to northeast vic at this time of year is well worthwhile - Bright Autumn Festival, Excellent wineries and restaurants and our great history and scenery.
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