It's not that the 'risks' of giving birth over 35 have changed - it's that in certain populations, it's become much more common than it was for a while.
100 years ago, most women who had children had many children throughout their reproductive life - like, 10 kids, first born in their late teens early 20s, last born in their mid-30s early 40s. That brings certain risks that have to do with how your body changes after bearing many children, and genetic risks. And certain benefits that have to do with how much help you have to raise the baby! Then once women gained greater ability to control family size, most women had several children within a relatively narrow window of their reproductive life - like, 4 kids, first born in early 20s, last born in early 30s. That leads to the 'nuclear' family which can be isolating for parents, but relatively few physical risks. Now, in upper socio-economic settings, many women are delaying childbearing until mid to late 30s. This brings certain risks that have to do with age and inexperience, and certain benefits that have to do with resources & maturity.
So where I live, for example, in a major metropolitan area, 'elite' doctors and midwives are quite used to women in their 30s & 40s having their first babies so may be less freaked out by it than they used to be.
I am 35 and will be when this baby is born. I know there are some increased genetic risks vs. when I had my previous babies at 27, 29, and 32, and I'm not exactly happy about them. But life is full of risk and unless I want to adopt (could never afford to) or stop at 3 kids, I have to accept those risks. I think DH and I would be able to rise to the occasion as parents to a special needs child, although I worry about how it would affect my other children. That's just a . . . magnification, though. I worry about how having another child will affect my other children anyway. (Not enough to not do it, though!) And I know that whatever child we have will bring gifts to all of us we cannot know about ahead of time, whether neurologically or physically typical or not.