A number of the behavioral symptoms⁷ of autism are observable by 18 months of age, including: problems with eye contact, not responding to one’s name, joint attention problems, underdeveloped skills in pretend play and imitation, and problems with non-verbal communication and language. 

Some studies also note that, although more subtle, some signs of autism are detectable at eight months⁸ of age. 
In general, the average age of autism diagnosis is currently three years old. In many cases, a delay in the child’s starting to speak around age two brings problems to parents’ attention, even though other, less noticeable signs may be present at an earlier age.⁹ 

Studies¹⁰ also show that a subgroup of children with ASDs experiences a “regression,” meaning they stop using the language, play, or social skills they had already learned. This regression usually happens between the first and second birthdays. 

Researchers are still learning about the features of regression in ASDs, and whether the features differ from those shown by individuals who show signs of autism in early life.