There are no blood or biologic tests for autism, so diagnosis is not an exact science. It's identified by making judgments about a child's behavior.
Doctors have been looking for autism at younger and younger ages, and experts have tended to believe most diagnoses are made in children by age 8.
However, the new study found significant proportions of children were diagnosed at older ages.
Dr. Roula Choueiri, a neurodevelopmental pediatrician at Tufts Medical Center in Boston, said she's seen that happening at her clinic. Those kids "tend to be the mild ones, who may have had some speech delays, some social difficulties," she wrote in an email. But they have more problems as school becomes more demanding and social situations grow more complex, she added.