Question: If an individual did not learn to speak in the first five years of life, is it a safe assumption that he/she has autism?
Answer: Many patients with autism have no verbal ability in the first five years of life. On the other hand, there are some children, for example, who have no verbal ability in the first five years of life but don't have autistic disorder. In order to make the diagnosis of autistic disorder, you have to have marked impairment in terms of language functioning, but, in addition, you need to have significant deficits in terms of social functioning, and you need to have substantial restricted interests and repetitive behaviors.
So just having difficulty in terms of language ability doesn't automatically give you the diagnosis of autistic disorder.