MORE Australian children are being diagnosed with autism.
BUT researchers don't know if it's because the condition is becoming more prevalent or if autism is being diagnosed earlier.
The Murdoch Children's Research Institute also found the overwhelming majority of children diagnosed with an autism spectrum disorder (ASD) are boys.
The development of 10,000 children was studied in two separate waves at two yearly intervals.
Half were recruited into the study at birth and the others in preschool.
When the children were 6-7 years and 10-11 years old respectively, parents were asked if their child had been diagnosed with an ASD and if so, the age and severity of diagnosis.
Quality of life, behavioural strengths and difficulties, vocabulary abilities and intellectual development were also measured.
The prevalence of an ASD diagnosis before seven was 2.5 per cent higher in the younger group compared with 1.5 per cent in the older one.
All parents reported their children had poorer quality of life and more emotional and behavioural problems than their non-ASD peers.
The prevalence of ASD of over two per cent for the younger group is higher than in previous Australian studies, said lead researcher Professor Katrina Williams.
"However, it is still not clear whether the difference in prevalence represents a shift to a younger age of diagnosis or a continued increase in diagnosis," she said.