Currently I think that anyone can "understand" (and that's the key word, not "know") something given enough time studying it.
But I disagree. Not matter how hard some people study they will never understand quantum physic fully. I think the difference is between understanding and learning. You can learn the facts, formulas, and material of a subjuect, but never really understand why they work or the hows of them to really get a deep understanding of the material. That is a factor of intelligence. Most anyone can memorize, few can really understand some subjects. And genetics plays a big role in this. Though there is something to be said about environment and upbringing. But I think their role is much smaller, and serve only to bring the potential to its fullest, but cannot exceed its limits. To go back to the athletic example. I am not a fast runner. If I train hard, I can certainly improve my speed, but I will never be close to an Olympic class athlete who has far greater genetic potential than I.