What? Your last post stated 'DD1 was never diagnosed as having autism'. I really don't get the point you are trying to make. Autism is a lifelong condition, if it is not present at birth it is not autism.
DD was extensively assessed in the specialist education system and nobody ever thought she had autism.
The neurodegenerative syndrome reared its ugly head and she suffered massive cognitive deterioration. As an adult she went to live in a specialist centre. They just assumed she had autism and it was in the paperwork they produced. I didn’t go round asking who made that diagnosis - the SALT department, the clinical psychologist or the consultant neuro-psychiatrist.
That is the nature of neurodegeneration - changes over time.
I don’t know how you can state categorically autism is only from birth; as if the areas of the brain responsible for the symptoms of autism are immune from the degeneration which affects areas like memory, IQ, language, aggression, irritability, walking and the development of psychosis?
They also don’t seem to know much about gene penetrance or expression? DD has 2 syndromes interacting. They don’t know why one of them is pathogenic in her; but is asymptomatic in most people with it. They can’t tell us, as carriers of one of them, how much the gene variant affects us; other than to say it depends on the penetrance.