IQ is a completely useless measure for this particular job. It measures acquired knowledge, explicit and inexplicit, memory, and processing power. Not universality. If someone is disabled to the point of lacking universality, then no, they couldn't learn Advanced Calculus. But yes, given enough time, and most importantly, actual interest, there's no reason someone couldn't learn it. The fact is, people like that have very very very little focus for things like that, because it's much more difficult for them and no fun at all. But if they for some reason became extremely interested in it and unlimited time, then yes, they could learn Advanced Calculus. There is nothing, in principle, stopping them.