I think we are tampering with nature and I really feel, having had children from my 20s to when I was 40, that your energy and your tolerance are really much better when you're younger. I think the ramifications of that might be very hard as the child grows into the teen-age years.
If the person has had other children it might mitigate the problems, but when you're set in your ways and a child comes, it throws you for a loop.
Another problem is that an older parent is much more likely to have physical problems. She might not have the stamina needed to raise an adolescent. She might become ill.
Emotionally you need a lot of flexibility and patience and the time to spend with a kid. You're much more likely to be successful when you're younger. The positive (aspect) might be that you're much more financially able to do for your child.