There has been considerable evidence that the plasticity of the human visual system persists beyond a so-called "critical period" in early childhood. The most famous example of this is the case of neuroscientist Susan Barry, who achieved 3D vision for the first time in her life at the age of 47 through optometric vision therapy. Her book about that experience, Fixing My Gaze was Amazon's fourth most popular science book of 2009. What vision therapy did for Susan Barry was an example of how visual rehabilitation is possible well into adulthood because od the plasticity of the visual system.
As another example, the National Eye Institute funded a study that showed that amblyopia can be treated in young adults, well beyond the critical period of early childhood which was previously thought to be the limit beyond which no change was possible.
A new study published in the Proceedings of the National Academy of Sciences of the United States of America examined a small number of patients who experienced early-onset (it began before age 1 year), extended-duration ( it lasted 8–17 years) severe visual impairment due to cataracts in both eyes. When the cataracts were removed and the corrective lenses implanted, the patients displayed considerable improvement in contrast sensitivity, which is a measure of vision ability. The study adds to the body of evidence that improvement in visual function can be achieved well beyond early childhood. The authors comment on the results as follows:
These results reveal that the visual system can retain considerable plasticity, even after early blindness that extends beyond critical periods.
However, it is still true that the earlier that vision problems are caught and treated, the easier it is and the better are the results. That's why it is recommended that children have an eye exam at six months of age and then every year thereafter. We still want to catch vision problems as early as possible.
Development of Pattern Vision Following Early and Extended Blindness Proc. Natl. Acad. Sci. U.S.A 2014 Feb 04;111(5)2035-2039, A Kalia, LA Lesmes, M Dorr, T Gandhi, G Chatterjee, S Ganesh, PJ Bex, P Sinha