Question: How does a parent's age at conception affect whether or not his or her child is at risk of having autism?
Answer: In general, the age of the parents does not have a major effect on the risk of autism, but there is probably a small effect related both to the age of the mother and the age of the father. The mechanisms are different. For older mothers, there's an increased chance that a child will be born with two copies of a chromosome from one parent, say the mother, and none from the other, say the father. And some of these conditions can be associated with autism. In the case of the father, the risk is for a new mutation, which is typically a change in one letter of the DNA sequence.
Increased risk associated with the age of either parent is nowhere near as substantial as in the case of Down Syndrome and the age of the mother. In addition, there's no easy way to test prenatally for these small increases in risks that might be associated with the age of the parents, in the case of autism.