Officially, AS requires both single words by the age of 24 months and communicative phrases by age 3.
Besides that language criterion, the two hugest diagnostic guidelines also say that those with AS usually have a special interest and that it would be unusual to have motor mannerisms (rocking, spinning, flapping hands) or being overly preoccupied with parts of for example toy rather than playing with it normally.
They allow a lack of special interest/a preoccupation with parts of object and motor mannerisms, but the wording is meant to urge the professional to use another dx because AS seems unlikely if those things are present.
Another definite criterion is that a person must had normal self-help skills (such as dressing and toilette training among others), normal adaptive skills (most of what's necessary to live independently) and normal curiousity about his or her environment (not being withdrawn, not being uncommunicative, not ignoring language or people and so on) to be diagnosed with AS.
In reality, all these are often disregarded and AS is dxed anyway and thus they're all not particularly meaningful in differentiating between HFA and AS.
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Autism + ADHD
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The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it. Terry Pratchett