Autism, by definition, can only be diagnosed if symptoms appear before age three. As a result, autism is usually diagnosed in children—often in children as young as three years old or even younger. Yes, there are circumstances in which autism is diagnosed in teens or adults, but the average age of diagnosis is between ages three and six.
Because autism is usually diagnosed in youngsters, many people think of it as a childhood disorder. In fact, most programs, therapies, and supports are available only to children with autism and their parents. But, it is incredibly rare for a child who is accurately diagnosed with autism to lose that diagnosis as an adult. The vast majority of children with autism grow up to be adults with autism.