Objectives: The purpose of this study was to compare the prevalence and case characteristics among three age groups of children with Autism Spectrum Disorders (ASD) residing in a single geographic area at a single point in time.
Methods: Information was collected on 4-year old children (born in 2002), 8-year old children (born in 1998), and 15-year old children (born in 1992) residing in three South Carolina counties during 2006/2007 as part of an ongoing, population-based multiple source surveillance of Autism Spectrum Disorders in South Carolina. Data were analyzed to compare the prevalence of ASD across age groups and to evaluate case characteristics including educational placement, developmental concerns, and cognitive ability.
Results: The overall prevalence of ASD was 8.0 per 1000 among 4-year olds (1 in 125), 12.3 per 1000 among 8-year olds (1 in 81), and 7.7 per 1000 among 15-year olds (1 in 130). Approximately 20% of 4-year old children with ASD had an educational classification of Autism, compared to 47% of 8-year olds and 58% of 15-year olds. At age 4, 56% of children with ASD also had intellectually disability (ID), compared to 49% at age 8, and 42% at age 15. Approximately 99% of 4-year olds had documented developmental concerns before age 3, compared to 89% of 8-year olds and 85% of 15-year olds. 60% of the 4-year olds identified in our study had an existing formal diagnosis of ASD, compared to 73% of 8-year olds and 70% of 15-year olds. Median age of diagnosis was 2.8 years for the 4-year olds, 4.5 years for the 8-year olds, and 8.7 years for the 15-year olds. There was a delay between the median age of first evaluation to the median age of diagnosis ranging from 1 month for the 4-year olds, to 13 months for the 8-year olds, and 39 months for the 15-year olds.
Conclusions: Our results indicate that the prevalence of ASD was highest among 8-year olds compared to the 4-year olds and 15 year olds. We speculate that this may be due in part to increased awareness contributing to better documentation among the younger groups, and to changes in special education eligibility across the lifespan. The declining incidence of comorbid ID suggests that children with cognitive delays may be recognized sooner than those without such impairments. Younger children had a shorter delay between first evaluation and first formal diagnosis, suggesting possible improvements in recent years. However, younger children were also less likely to be served educationally under an Autism eligibility, suggesting possible delays in access to appropriate educational services despite early identification.