From memory, Mum was 28 I think Dad was 32. I think the age thing is clutching at straws so far as a reason for ASD.
Research is now leaning towards bioflora being a contributing factor; so diet, whether or not you were born naturally or by caesarean, and how much exposure you had to antibiotics. It seems likely that the bacteria in our mouths and gut play a strong contributing role towards obesity, diabetes, depression, and ASD. How much is about this and how much is other factors is yet to be determined, but clearly this seems plausible.
I still believe that in time ASD will be shown to have numerous interacting factors which contribute to whether it occurs, as well as severity.
Some will be biological, some will be environmental, and some will be a combination of both.
I very much doubt that one single cause will be shown to be more contributory than another, but can stand to be proven wrong.