According to the WA education website:


So your DD will need to start Grade 1 in 2012, but Kindergarten & Pre-Primary are not compulsory.

Early Childhood Education
That site has the information about the maximum age too - go down to the " Comparison of ages for entry into Programs in Australia " and for WA its the beginning of they year where the child will reach the age of 6y 6 months

Going by the ages for the rest of the states (wher the year before Year One is compulsory) this is the max are for pre-primary entry