happyfacegrin you are correct, you can enter the IDP system as late as IDP 5, which is normally competed at around 9 years of age, but the system is not really set up to encourage this. A talented child may transfer to IDP 5 from the national level stream, but usually if they have not previously trained in the IDP system, at least done IDP 2 and 3 they don't tend to have the skills that are necessary. Most gyms do not have the ability, coaching expertise or facilities to develop elite gymnasts or even set the basis for doing so. Therefore, if the kids have not been identified earlier it would be highly unusual for a child to go from a national path to an international path as late as 9. Our system does work for our country, it can be a little exclusive, which may work to our detriment but I don't think we would have a fraction of the international success we do have without it.
I think one big difference is that we do start competing gymnasts at level 1 when they are 5-6 years of age. The USA also starts competing kids at age 6, but in level 4. I have never in my life seen a 6 year old compete level 4 in Australia, our routines are not designed to encourage it. Our Level 4's must jump to High bar and go over the vault table at at least 105 cms, the US don't have these requirements. If we had a child capable of level 4 standard at 6, she would be in the IDP stream.
On the original topic, I do prefer Australia's national stream system, where from level 4 each gymnast has their own individual routines and floor music with a set of skills to be built in. It helps to keep competitions interesting and exciting (for competitors, coaches, spectators and of course judges!), it does make for more effort on the part of gyms having to choreograph all individual routines from level 4 though.