I don't know those other guys, but I can say Jacob Barnett is a PhD student, right now. As far as I'm aware, it would be seriously jumping the gun to say he was a flop at this point, just because he hasn't been in the news for a year or two. Historically, there have been a lot of child prodigies who were very successful, like Norbert Weiner, Mozart, Gauss, and fairly recently, Terence Tao. Your whole idea of people who can think versus people who can only learn suggests a certain naivete about what it is like to actually do research. I think it's almost more a matter of having a strong stomach, so to speak, than being able to think, in my experience. Or 99% perspiration, as Edison put it. Of course, it helps to be really interested in your work, so that you don't have to have quite as strong of a stomach to make it through. Someone who does a PhD in math or physics or even gets into the program is generally relatively good at thinking. Not everyone can solve those textbook problems. It's not just a matter of being able to learn and not think.