I teach year 1/2 at the moment and they are doing short, sharp reading lessons that are 10 minutes in groups of 2 or 3. If a teacher is teaching anything for 45 minutes they will lose the students whether they are 4 or 44.
It's interesting you mention the recent project segment as the guest they had on the show, Dan Haesler who is a teacher is a friend of mine who advocates for children starting the year they turn 6.
If a child turns 6 in the first year of school, they will turn 12 in the last year of primary school.
Not sure what you mean about being "kept" but out of the top 10 ranked oecd countries in education, all have a later starting school age than Australia.