Education - NSW ought to just about cover it Shannon - as you can see there will be about a disparity of a year in ages in any one class for one could have turned 6 in December and been 5 years of age all grade 1 whereas another could be nearly a year older in being 6 all year if turning 6 in January.
So if your Jnr. is turning 9 next year, then it is likely he will be in grade 4, but there is possibility that a school will take into account what his schooling has been to date and if he is finishing or half way through grade 4 already, there could be a case you can present for him to be considered for grade 5 or alternately he start in grade 4 and perhaps an assessment of his ability could be made in the first month or so, and a possible move to grade 5 being considered.