I've been pondering this question for a few days now. Personally, I think it depends on the individual(s) invovled. I wouldn't want to have children past my 30's, but that's just me. As long as the mother is healthy and seeks prenatal care to ensure she and baby are doing ok, then to each her own.
I also want to add food for thought. Being married, financially stable, etc. are wonderful things to want and have before giving birth. However, there are no guarantees that things will "work" when we want them to.
I hate to say my sister and brother-in-law made a "mistake", but they waited for that "just right time" to begin a family, and it never worked out for them. She had three miscarriages (I think it was three)
, and tried some other methods. They finally adopted their first and only child three years ago, and they are in their early 40's. Now that I'm married, she keeps telling me not to wait around too long "just in case" because the older we get, the more difficult it can be.