Well, hang on - there are actually learning disabled people who are incapable of learning to tie their own shoes. That's an extreme outlier, of course, but it suggests that there are two possible interpretations for "intelligence": one, that everybody has an upper cognitive limit that they'll never be able to surpass, and the mentally disabled are just struck with a very unfortunately low limit. The other is that being mentally disabled is like being crippled: you either are able to walk/run (learn) or not, and it's just a matter of how much effort you expend in improving it if you have it. Even then - we know that some people learn much faster than others, so that would define intelligence as a measure of how quickly you can absorb new concepts. You seem to be leaning toward the feel-good, but very provably wrong, "everybody is as smart as everybody else" (captured by the Facebook-popular "everyone is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid" quote falsely attributed to Einstein), but you're not doing anybody any favors suggesting that it's going to be as easy for them to learn calculus (if they can learn it all) as it was for you.