It all depends on your overall health, your mental state, your personal circumstances, and genetics. There are people who can have a healthy baby and be healthy themselves in their mid to late 40s, there are those who can't conceive after 35. The only way to know if you're still fertile is to have tests done. And even then, it is no guarantee.
I had my last child at age 36 and I almost died, literally. The baby was premature and it was touch and go for us for the first month. That pregnancy and birth kicked my butt. But going into it, I was in what people consider excellent shape and health.
When it comes to the actual "mothering" part, I can honestly say I don't have the energy or same interests when it comes to baby time, playing, etc. as I did when I was younger. It does get harder both psychologically and physically. You will also discover that it is very hard to find other moms your age with young babies to socialize with....which as your child gets to be of an age where playdates are important, is frustrating. Sure there are lots of moms with young babies, but not that many who are your age, with similar shared challenges and problems because of where you are in life.
There is a difference between someone who has had an established career starting a family and someone who has chosen to do so as their primary vocation (no rub on anyone, but it is cause for isolation for many an older mom) It's a big deal...more than people will let on. All of my peers (friends I grew up with) had their kids in their mid to late 20s or have chosen to have NO children at all but to focus on career instead. I started in my mid 30s. So all of my friend's kids are way younger than mine, with the exception of two who aren't very close, making socializing more challenging and seldom because we no longer have much in common.
I have a friend who has been trying to conceive her first child. She waited because she wanted to be financially fit. Well, her finances are mint and her career settled at age 43. After trying for several months and a terrible miscarriage, she just found out she's probably going to have to adopt.
Statistically, fertility goes down after 35 and health risks for both baby and mom go up. While anything is possible with faith and science, I say the sooner the better for health, social, and sanity reasons.
Don't get me wrong...having kids is a wonderful blessing. It's just challenging no matter what your age is, and even more so when you choose to be an older mom with young children. And even then, for first time moms or those wanting to add one more, there are no guarantees you will be able to have anymore without medical intervention or without medical challenges.
With all that said, if you really want another child, go for it. I think Dr. Oz isn't saying not to have them, I think he's saying women need to be realistic about their options. Fertility doesn't last forever and he's telling women to plan accordingly if family is important for them. I think people need to think about it. We put so much energy into career and take family (conceiving one) for granted. It isn't a right, it's a privilege. If you want children you only have a small window of time and also need to keep in mind the sacrifice it entails from the potential physical challenges, financial, and emotional investment. It's a big deal, but for some reason so many people put it on the level of buying a house or car. There is always another house or car around the corner, but not necessarily another opportunity to have a child.