My Sister in law was recently diagnosed Autistic at aged 31 how is this possible?

I don't understand. We know she has anxiety disorders etc and PTSD. But autism? At her age! how could it be missed for that long. I definitely think it's inaccurate. She would of been diagnosed in school. I've told her to ask for another opinion. What do you all think.

My Sister in law was recently diagnosed Autistic at aged 31 how is this possible?
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