Is it possible that a 17 year old girl got diagnosed with autism and the parents never noticed anything before?

Answer 1

There are adults being diagnosed with autism like Daryl Hannah, and Dan Akryod. So it's possible not to notice until they are 17 years old. 

Answer 2

 If these “parent”s were never involved with the child zxj

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Answer 3

No, not really.

Answer 4

If she was on the mild side of the Autistic spectrum then it is possible that they didn't notice or just put it down to her personality.