If your child is over three years old, diagnosticians will ask whether your child's symptoms appeared before the age of three. If the tests are administered correctly to a child whose symptoms appeared before the age of three, and her symptoms fit the criteria for an autism spectrum disorder, then a diagnosis on the autism spectrum is appropriate.
It's important to note that for the purpose of diagnosis, it doesn't matter WHY the symptoms exist. Today, in about 20% of cases, the causes are well known. Known causes of autism can include prenatal injury, injury at birth, hypoxia, mitochondrial disorder, genetic disorders, and more. In short, if it looks like autism - and the symptoms came on prior to age three - then it's autism.
What if a child is injured at age four and acquires the symptoms of autism? In that case, according to the official diagnostic criteria, the child is too old to be accurately diagnosed with autism. In other words, if a child is injured at age two years and eleven months, and acquires autistic symptoms - he's autistic. If he's injured at three years and one month, he's not.
Interview with Dr. Susan Levy, Director of the Children's Hospital of Philadelphia Autism Center, November 2009.