One in four children (26%) were found to have delayed school entry, with boys, younger children, those with an Australian born mother and socio-economically advantaged children more likely to delay. The tendency for children to start school when they are first eligible clustered in disadvantaged urban areas. 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 percent in the probability of the child scoring above the 25th percentile in all five AEDC domains. Also, regardless of school starting age, children who were older in relation to their classroom peers had better development outcomes. Initial age-related differences may also impact on later school outcomes.