According to the Centers for Disease Control and Prevention (CDC), while there is more than one cause of autism spectrum disorder (ASD), most of the causes are still unknown. There is a wide variety of potential symptoms that someone with ASD might display, such as a lack of interest in other people and high sensitivity to sound. ASD, which now comprises autistic disorder, pervasive developmental disorder not otherwise specified, and Asperger syndrome, often begins before the age of 3 and will last a whole lifetime. The CDC says that symptoms could begin as early the first few months of a child’s life. The organization also lists a variety of potential symptoms, such as, “not responding to their name by 12 months of age, not playing ‘pretend’ games by 18 months, and getting upset by minor changes.” While most people with ASD have difficulties with social skills, their ability to effectively communicate and speak differs greatly depending on the person. Other symptoms, such as hyperactivity, aggression, and temper tantrums, also could be signs of ASD.

Source: https://www.cdc.gov/ncbddd/autism/signs.html