It really depends on the woman. Some women start to have trouble after 35 and some have no issues at all. There's soooo many factors involved so don't start worrying about the statistics too soon.
I am 38 and I have been TTC #2 since last November. I definitely think that age has been a factor for me. However, the more I learn about it the more I think that age is not really an insurmountable barrier. From what I've been reading it's not age in and of itself but some of the things that usually happen as we age that is the problem. As we age, less blood tends to flow to the reproductive organs. Also, hormonal changes and long term nutritional deficiencies can play a part. I think once these are addressed, age shouldn't stop a woman who is still able to ovulate and has a uterus from getting pregnant...Or at least I'm hoping!!