Clinical data in Japan were used to examine the age at diagnosis of autism spectrum disorders (ASD) in children in Japan. Results indicate that the mean age at diagnosis for 7779 participants was 7.3±4.3 years, the mode 3.0 years, and the median 6.0 years. The male-to-female ratio was 3:1. There were no sex differences in the age at diagnosis of any ASD. Asperger's syndrome was diagnosed at older ages than other disorders, but no clear peak was indicated for the age at diagnosis. Only childhood autism was diagnosed early in life and showed an annual trend. This study suggests the need to develop a system or tool that enables early diagnosis of Asperger's syndrome, as well as the need to develop a proper early intervention techniques and service provisions for childhood autism.

Behavioral problems directly affect the quality of life of caregivers and children with autism spectrum disorder (ASD) and/or attention‐deficit/hyperactivity disorder (ADHD), and is known to be associated with clinical factors such as gastrointestinal (GI) symptoms, sensory abnormalities, intellectual abilities, and use of medication. However, previous studies have not considered these relationships comprehensively. We conducted a cross‐sectional study of 6–12‐year‐old children with diagnoses of ASD and/or ADHD at two hospitals in Japan. Scores for the aberrant behavior checklist (ABC), autism‐spectrum quotient (AQ), and Conners 3, as well as information on daily sleep and exercise, GI symptoms, and Short Sensory Profile, were collected. Each factor was subjected to a correlation analysis to investigate its effect on ABC scores. A stepwise multiple linear regression analysis for the factors with p < 0.05 was performed. Data were obtained from 60 patients with a mean age of 8.3 years; 21 had ASD alone, 18 had ADHD alone, and 21 had ASD + ADHD. The correlation analyses identified six factors associated with ABC severity: (a) methylphenidate use, (b) Conners hyperactivity score, (c) Conners inattention score, (d) AQ score, (e) SSP score, and (f) GI symptom score. The multiple regression showed that “GI symptoms” and “sensory abnormalities” were independently associated with ABC severity. Although further studies are needed to show a causal relationship, appropriate assessment of GI symptoms and sensory abnormalities may help alleviate some problematic behaviors and improve the quality of life of children with neurodevelopmental disorders and their families. Lay summary Behavioral problems in children with neurodevelopmental disorders are known to be associated with many factors. This study aimed to comprehensively investigate the known factors. We have discovered that “gastrointestinal symptoms” and “sensory abnormalities” were independently associated with Behavioral problems. Our results suggest that it is important for clinicians and caregivers to pay more attention to children's GI symptoms and sensory abnormalities that may not present as obvious symptoms or complaints.