Relative age effects (RAEs), where older children within an age-based group have better academic, sporting, self-concept and mental health outcomes than their younger peers, have been demonstrated in multiple countries. Australian research has found self-concept and Attention Deficit Hyperactivity Disorder RAEs, but academic RAEs have not been explored. This cross-sectional observational study investigated RAEs among Western Australian year 3, 5, 7 and 9 government school students who took a National Assessment Program - Literacy and Numeracy (NAPLAN) test in Numeracy, Reading, Spelling, or Grammar and Punctuation in 2017. After adjusting for gender and Aboriginal and Torres Strait Islander status, we found that among the 98% of students who were in their recommended school year, younger students were on average significantly outperformed by their older classmates in all tests. RAEs were strongest in year 3, and progressively weakened through years 5, 7 and 9. These results are consistent with international research. They also support the hypothesis that age-related immaturity of relatively young children disadvantages them in academic competition, but this decreases as the age difference becomes a smaller proportion of their lived years. We also examined the performance of the 2% of children who were outside their recommended year. Accelerated students on average significantly outperformed their older classmates. Delayed progress students performed significantly worse than their younger classmates. A likely explanation for this reverse RAE is that accelerated students are often promoted because they are high academic achievers and delayed children are held back because they are performing below expectations.