Diagnosis:
Research has shown that a diagnosis of autism at age 2 can be reliable, valid, and stable.
Even though ASD can be diagnosed as early as age 2 years, most children are not diagnosed with ASD until after age 4 years. The median age of first diagnosis by subtype is as follows.
Autistic disorder: 3 years, 10 months
ASD/pervasive developmental disorder (PDD): 4 years, 8 months
Asperger disorder: 5 years, 7 months
Studies have shown that parents of children with ASD notice a developmental problem before their child’s first birthday. Concerns about vision and hearing were more often reported in the first year, and differences in social, communication, and fine motor skills were evident from 6 months of age
— Yet DSM III had this requirement “Onset before 30 months of age”
So basically most children diagnosed today would not fit the DSM III criteria, which means that in the 1990s they would not be classified as autistic and the insurance would not cover for their specialized care.