Then why is the ‘environment’ of a 40yr old mother different from the ‘environment’ of another 40yr old mother?
Unless the argument involves some sort of protective mechanism of early youth pregnancy ‘protecting’ the eggs somehow. In that case it’s ‘environmental’ and related to the chemical/biological process in question. And not age related.
Genetic susceptibility. Most of the epidemiological studies are pointing at an interaction of genetics and environment.
There is no getting around the data on age. A mother over 40 has a 77% higher risk of having a child with autism than does a mother under 25.
If age was irrelevant, that risk differential would not exist or would be so small as to be irrelevant. Maternal age has no effect on color blindness or hemophilia, but infant gender does because they are x-linked recessive disorders.