It is the father's age that causes spontaneous mutations that cause autism, etc. not the mother's

By age 27 a man's sperm has divided 300 times One new mutation in sperm. each time it replicates.

Geneticist James F. Crow wrote in 1994: "I don't find this nonlinear effect at all surprising. Everything gets worse with age, so I fully expect fidelity of replication, efficiency of editing, and error correction to deteriorate with age. For a man of age 20, the male mutation rate is about 8 times the female rate. With a linear increase, in a man at age 30, the ratio is 430/24 = 18, at age 45 it is 770/24 = 32. With nonlinearity, these ratios are much larger, some 30-fold at age 30 and as much as two orders of magnitude at age 40. Examples such as MEN2A, MEN2B, and Apert syndrome, in which a total of 92 new mutations were all paternal, are therefore not so surprising. Whatever selective forces reduced the mutation rate in our distant past, at a time when most reproduction must have been very early, were not effective for older males.
I conclude that for a number of diseases the mutation rate increases with age and at a rate much faster than linear. This suggests that the greatest mutational health hazard in the human population at present is fertile old males. If males reproduced shortly after puberty (or the equivalent result were attained by early collection of sperm and cold storage for later use) the mutation rate could be greatly reduced. (I am not advocating this. For one thing, until many more diseases are studied, the generality of the conclusion is not established. Furthermore, one does not lightly suggest such socially disruptive procedures, even if there were a well-established health benefit.)"