There are many forms, and degrees of autism. Two of which are classic autism and autistic syndrome.
Classic autism is the more severe of the two.
It can include lack of communication and even self-injurious behaviors. This form of autism is usually noticeable within the first few months of life. Autistic syndrome is harder to notice and can go undetected for the first two years. These children do not present symptoms that most doctors would find abnormal. Up until the age of onset these children can appear normal.