This statement, older women run a higher risk of bearing an autistic child, really means, "More children born to older women (than to younger women) are diagnosed with autism." However, we do not know how many children are born with autism (or "autism spectrum") and are not diagnosed. See my post above for some reasons why the child of a 30- to 40-year-old mother might have a greater chance of being diagnosed than an identical child born to a younger or older mother.
Since we do not know what causes autism, we cannot apply the reasoning under which age is causative for some conditions. For instance, we know that ova tend to degenerate with age, making maternal age a cause of a higher risk of babies with Down Syndrome. We know that tiny, benign colon polyps grow and deform into large, malignant tumors over time; thus, age is a cause of colon cancer diagnoses.
We do not have sufficient knowledge of autism to make any sure statements, in my opinion. The "nerds mating" hypothesis would also explain a correlation of autism diagnoses with older mothers: the highly-educated mother (the bookish sort who didn't have boyfriends in high school) marries the socially-akward IT-guy whose economic value has suddenly skyrocketed. Is their child more likely to be autism-spectrum because of their genes? Is it her age? Is it her (physical) age or her (as it were) life-stage conditions? We don't know.
We do not have sufficient knowledge of autism to make any sure statements, in my opinion. The "nerds mating" hypothesis would also explain a correlation of autism diagnoses with older mothers: the highly-educated mother (the bookish sort who didn't have boyfriends in high school) marries the socially-akward IT-guy whose economic value has suddenly skyrocketed. Is their child more likely to be autism-spectrum because of their genes? Is it her age? Is it her (physical) age or her (as it were) life-stage conditions? We don't know.
While there are many influencing factors, each of the above you mentioned influence the mother's ability to emotionally bond to the child. Bowlby and Ainsworth were more correct in their research than even they imagined.
But I think the issue linking “nerds marrying nerds” and all the more specific reports linked to that results from the classic element of change in input causing change in output.
Women have been older mothers for years. When “pioneer” women had 10-12 children, spread out over 20 years, there was appreciably less autism.
The change was men and women from the same specific disciplines marrying each other much more frequently. Women who were strong in math met and married men they met in math graduate school. Lawyers, with the mental skills required form that profession, met in law school and married each other, as opposed to a school teacher or nurse.
That was the change in the input, not parental age.
Remember, Autistics are often brilliant in one area, but totally deficient in all others. Are their areas of brilliance from the same input from both parents? I think so.