[sthlm]

What you're talking about is known generically as the General Learning, which is an aptitude in itself. Some people learn better than others. It also depends on what you learn.

Nurture: At various ages, the ability to learn changes (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.88....). It might well be true that there is a lot of optimization to be done in the way we teach -- something that Salman Khan illustrates well with his new teaching methodologies. That doesn't necessarily mean however that at all stages, ages, circumstances, people will be equally able to learn something.

Nature: Some people have an innate aptitude; I won't look for papers, as I find this to be truly obvious.