I am an accredited ASD Assessor. The National Guidelines say a child should be seen by a paediatrician first. If a paediatrician is confident of the diagnosis, they can do an assessment without involving a second assessor. It takes 2 appointments. If the paediatrician is not sure, then a referral should be made for a dual assessment (Usually a speech therapist and a psychologist). Assessments can be done from 2 years of age. I don’t tend to assess teenagers. However, in special circumstances, I will do the second assessment with a speech therapist.
Many families are asking about Pathological Demand Avoidance. https://childmind.org/article/pathological-demand-avoidance-in-kids/
“While some experts are pushing to recognize PDA as a standalone diagnosis, or a subset of symptoms of autism, others feel it is too vague to be clinically helpful.” I am very familiar with the presentation but do not find it is needed as a stand alone diagnosis.