I think if you are experienced in knowing what to look for, you can spot it pretty early on like some people can tell before the child is a year old. It is most commonly spotted around 18 months to 2 years when they should be starting to talk. in some cases, when the symptoms are fairly mild like in Asperger's, it can take longer because speech may not be delayed and the child may seem in most ways to be developing typically.

They typically say by the age of three when they would really be interacting with others with purpose. but if your child has a speech delay at about 1 or 2 autism might be the reason.

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Depends on what kind….
Sometimes as early as toddlers because they don't play correctly. some kids are unable to social interact. Sometimes in Kindergarten when they start school and they are not learning at the correct speed. my nephew was 12 months and they new something was wrong with him but they were sure if he was retarded or it was autism.
But Asperger's Syndrome sometimes isn't detected until high school!

They dont start to talk.
They dont interct, (smile, show emotion) (excluding crying, thats totally normal)
Anything that is out of the ordinary, go see your docter, then if your docter iis concerned he/she can provide you with a perscription with an autistic specialist.

I would say even as early as 13 months, my son was diagnosed at the age of 4, but there were signs from that age (13 MTHS) that there could be a problem, his speech didn't develop at all, very bad tantrums, lining things up all day and not really playing or doing anything else exept that, also doing things repeatidly, oh and his speech didn't develop however he would say 'washing machine' all the time in answer to every question.

It really depends on the type, some people have regressive autism, which means that they develop typically until the age of 15 to 24 months and then lose skills. some people have had unusual characteristics since birth, and the University of California has developed a survey for detecting autism as early as four months old.

Aspergers is a type of autism, that often is not discovered until grade school, because the primary distinction is that there are NOT early delays.

But with typical autism, the average age of diagnosis is around three or four. often parents detect troubling characteristics before the age of two, however, and it takes some time to go through the medical channels for a proper diagnosis.

HTH.

Usually the symptoms can be spotted out from a doctor from a very young age. unfortunately I do not know the exact age.