The selective test is conducted at year 6. Normally students are at 11 years old. Many students are only 10-10.5 years old because parents put them to schools a bit too early. Some students are already 12 years old or at least nearly 12. This means puberty would have started. At this stage (according Jean Piaget), they enter the
formal operation stage of cognitive development. The main difference is that they start to be able to handle abstract concepts. This is exactly the reason why Western countries tend to start formal mathematical algebra and scientific method at year 7. Piaget's theory is one of the most influential theories in Western education.
When students are still under 12, it is unlikely that they have started puberty and this means they have limited ability to understand abstract concepts. The selective test and scholarship tests are staged at year 7-8 level both in reading and mathematics with a lot of abstract concepts. So this favours older kids. This does not mean immature kids will not do well. They do ok but will have to work a lot harder to compete. This is why it is so important for parents to start their kids late or at least right on time (6 years of age at school year 1). It is best to have the kid nearly 7 years old at school year 1. This maturity is an enormous advantage. A lot of middle class Australian parents are experts at this. They start their kids early in preschool, hold them back for 1 year and start kindie very late. However, this is mainly for advantages in sport and leadership at schools.
The English part of selective test is really positioned at year 7-8 level with questions exploring abstract thinking. This is why most kids score between 40%-50% even though they are rated B+ to A grade in English at schools! This is why 85% in ICAS English competition only matches 55%-60% in selective English (scholarship English is positioned at similar level like selective English).
How do Singapore teachers deal with immaturity when they teach algebra? The answer is simple. They move the method back to a stage earlier in Piaget's theory (concrete operation). They use concrete objects such as blocks, scale and diagrams to teach albegra. This is extremely effective as illustrated in the Math Problem Solving Guide.
What should parents do? Clearly delaying the start for students is the best way (kids start kindie in Finland at 6 years old; Singapore has 2 kindie years; South Korea starts year 1 at 7 years old). These are among the countries with best educational performance in the world.
What to do with kids who are immature when you cannot hold them back 1 year? Australian schools will not allow this! Parents will have to share the burden! Parents will have to read stuff and watch movies ... together with kids and feed them with ther perspectives of an adult and try to get the kids to understand the adults' views. It will be many times as hard for them to understand (when compared to kids 1 year old who have started puberty).
One comment from a parent who visited Sydney Grammar (at primary school level) was that the kids looked so mature. She chatted to them and found out that they were mostly 1 year older than expected. As Sydney Grammar picked students by a rigorous selection process. Unintentionally (

), they have been picking up the older kids who could score well in the tests. This is a common thing at top private schools. They have older students either because parents delay school, change school and repeat some kids 1 year, or the selection processes favour older kids.
How do you test a kid for ability to understand formal operation?
Ask the kid about an abstract concept like "What is a home?"
Kids in pre-operational stage (2-7 years) will probably answer: a house.
Kids in concrete operational stage (7-11 years) will probably answer: a house where a family lives.
Kids in formal operation stage (11-15 years) will probably answer: any place where members of a family can be permanently together.
You see that the concept of a "house" has been abstracted into a "place". This place can be a house, a unit or any place where a family can be together in a permanent basis. Sadly, for some people, this "home" is just a shipping container in the Australian Nauru detention centre. The older kids around late 11 years or 12 years of age are capable of this level of abstract and deeper thinking. They would generally outscore the younger kids in English comprehension.
Try to ask kids what a "school" is and you will get different answers. The older they are the more thoughtful the answer is. But there is likely a clear change around 11-12 years of age.