The average persons capabilities.

[robvalue]

I think people can improve at just about anything, with practice and tuition.

But I think the amount of time it takes people to improve at any particular thing varies enormously. So it may not be practical for a given person to train up to the required level to do a given job, even if they could theoretically get there eventually. I would say there would almost certainly be something they could train to do, though.

I feel there are "soft ceilings" in place, beyond which progress might not be impossible, but becomes extremely arduous. I think this is to do with the natural aptitude for the kind of task.