School starting age and child development in a state-wide, population-level cohort of children in their first year of school in New South Wales, Australia

In Australia's most populous state, New South Wales (NSW), children born between January and July have the choice to start school in the year they turn five, or delay entry until the year they turn six. We used linked administrative data for children who started school in NSW in 2009 or 2012 (N = 162,878) to identify child, family and area characteristics associated with delayed entry, and to explore the relationship between school starting age and five domains of child development, measured using the Australian Early Development Census (AEDC) in the first year of school. Among both the 2009 and 2012 cohorts, 26% of children delayed starting school until the year they turned six. Area-level rates of delay ranged from 8% to 54% across 198 areas in NSW, with lower levels in disadvantaged urban areas. Factors associated with delayed entry included male sex, a birth date close to the enrolment cut-off date, socioeconomic advantage, and having a mother born in Australia. There was a strong, significant relationship between school starting age and early childhood development: each month of maturity corresponded to an increase of approximately 3% in the probability of scoring above the 25th percentile in all five AEDC domains. Independent of school starting age, children who were older in relation to their classroom peers had better development outcomes. The potential for initial age-related differences to impact later school outcomes warrants further longitudinal research.