I was a terrible student more interested in playing games than doing any actual work, so that I'm sure was a large part in the problem being "years" instead of "months", but either way it was something I learned and not something I was just born with.
I do agree that for any person there will be things that they get right away and things that they'll never get. But I think those vary more than people realize.
End of the day, learning its a personal thing, and some people struggle in some topics and not in others, but under no circumstances do I think there's an innate ability to understand pointers and some people are never going to have it. That just sounds like C programmers trying to pet their ego.
Have you thought about may be you don't understand pointers and just think you do ? May be you should not start teaching and explaining about pointers before you understand it well. If you can't pin-point why your students don't understand and just say they won't understand at all (may be weak math background or something else), then I'm sorry you lack the knowledge to teach or use pointers.
I'd like to believe that's true, but I've spent quite a lot of time trying to explain pointers and recursion to people and either they get it right away or practically never. Sometimes they end up with some cargo-culted half-way understanding that lets them solve most problems but they still don't seem to understand what's happening. Joel is a bit pompous, that's true, but there definitely are people who cannot understand pointers when taught in a normal CS course, and if 4 years of university didn't teach them pointers I don't think they'll get them while programming in the field.
As an aside, the fact that some people can't grok some things shouldn't be controversial. I myself can't understand lots of things and probably would never understand them in the way that people who grok them instantly do. People have different modes of thinking, and I do not doubt that there exist people who cannot grasp pointers and recursion in the same intuitive way that I do. That doesn't make them less smart in general, but it does mean they're a bad fit for e.g. a C programmer position.