It totally depends on the genetics of the people involved. Some women might safely be able to have kids until their late 40s, and the eggs will be fine, the kids will be normal. Halle Berry had her child naturally at 42, but then again, she had the baby with a young man, which usually makes a difference.
Sometimes it makes no difference. John Cleese’s parents were both in their 40s when they had him, and he was a surprise. David Letterman was ~60 and his now-wife in her mid-40s when they had their healthy child, and he was a surprise as well. Some women’s reproductive systems peter out early. Some women’s eggs go bad “early”, as does some men’s sperm, making for miscarriages, fetal abnormalities and other problems.
In other words, it’s a crap shoot, but the odds favour some complications past 35 and definitely past 40 for most women.