I believe what is important is not how much you know about a certain topic but how much you want and need to know about that topic. If you learn a lot about a topic but you don’t understand its usefulness to your particular goals, than all that you know about that topic will probably be useless or trivial.
On the other hand, if you knew only a little or nothing at all about a topic, but you did understand its usefulness to your particular goals, then you will be much more likely to learn all that you need to know about that topic.
I believe we all have the ability to learn whatever we want to learn. However, I know from my own experience that I have a really hard time learning what I don’t want to learn.