I don’t know about birth defects, etc. being more likely the older mothers get, however, I’ve heard that fertility declines in both men and women as they age.
So, it seems that women trying to get pregnant after 35 could have more problems getting pregnant than they would when they were in their 20s.
This brings up a point, that, for people going for professional careers, they have to be willing to work hard, work late, etc. in their 20s, to build the foundation for that career. And that is difficult to do, if you are a mother at that age, with small children at home.
to me, as I’ve learned, being secure financially is a must and thing is that young men are showing themselves to be increasingly unreliable. They might enjoy being married for a while, but the time sometimes comes when they might realize that it ‘just doesn’t work’ and then they bail out. So the woman is on her own with no job skills and a kid and struggles.
Or, the woman in the marriage marries a man to escape and then leaves him for a richer man in the future. People in their twenties are too volatile and I’ve seen too much in mine to want to marry before I have my own security.