If you were old enough to have "delusionary imaginary friends," you were definitely
not a baby. Probably in the preschool age range at the earliest. To have an imaginary friend, you have to be old enough to understand the concept of friendship.
It was not that long ago that autism was rarely diagnosed because:
A) Parents would do anything for a different diagnosis because autism was always blamed on "
Refrigerator mothers."
It was only a few decades ago that cold, unloving mothers were blamed for their children's autism. Read more about this debunked theory.
www.verywellhealth.com
B) Unless it was really bad, mental health pros didn't have a clue what to look for. High-functioning autism wasn't a term in use back then. Maybe a passing mention of Asperger's they promptly forgot about because it wasn't considered serious enough.
So moderate levels of autism got diagnosed as other disabilities.
Today autism is a very common diagnosis. Still, even today there are a lot of poor and rural (and often culturally conservative) areas where it doesn't get diagnosed. There are still shrinks who deny females can have it. That's just what they learned.