I came across an interesting study the other day regarding presbyopia: http://www.nature.com/articles/srep00278
Essentially what happened in the study was they took a number of people who normally wore reading glasses for presbyopia, and had them undergo a sort of vision training. The training involved intensively focusing to try and resolve minor differences in color contrast (from about 2 feet away). After several months of training, the subjects' presbyopia was significantly improved, and all of them were able to read a newspaper without reading glasses, something that they could not do before.
The scientists did some optical measurements of their eyes before and after, and didn't observe any changes with the eyes themselves. Therefore, they concluded that the difference must be with how the brain processes the images that the eyes received. That is, the brain become better at "deblurring" images.
I found this an interesting study for a couple of reasons. First, it shows that it is indeed possible to overcome presbyopia to some extent. Second, it raises the question: are improvements in vision like some on this forum (myself included) have experienced due changes in the eye itself, or rather in our brains' processing of the images it receives? Or maybe both? The subjects' accommodation (which didn't change) was measured, rather than axial length directly....but that might amount to the same thing. I'm not sure on that.
If the improvements in vision are purely neural, then that suggests to me that there might well be a limit on how much improvement can occur. I'd like to believe that's not the case, though.