The study found that the incremental risk of having a child with autism increased by 18 percent -- nearly one fifth -- for every five-year increase in the mother's age. A 40-year-old woman's risk of having a child later diagnosed with autism was 50 percent greater than that of a woman between 25 and 29 years old.And as to why:
One possible clue comes from a 2008 UC Davis study that found some mothers of children with autism had antibodies to fetal brain protein, while none of the mothers of typical children did. Advancing age has been associated with an increase in autoantibody production. Further work investigating advancing age in such findings may be useful, the study authors said. They added that some persistent environmental chemicals accumulate in the body and also may have a role to play in autism, possibly contributing to the apparent effect of parental age.
The study also suggests that epigenetic changes over time "may enable an older parent to transfer a multitude of molecular functional alterations to a child ... thus epigenetics may be involved in the risks contributed by advancing parental age as a result of changes induced by stresses from environmental chemicals, co-morbidity or assistive reproductive therapy."In short, it's god-awful complicated, and they're working on it.