I'm no vision expert, but am watching a friend's 10-year on his 3rd glasses' prescription in four years, and they tell me this rapid deterioration starting around age 6 is the usual pattern in his father's family. So yes, (based on this n of 4!), vision could suddenly drop off.
Also, do a quick search on this forum on vision therapy. There's been quite a few of us with kids who passed normal vision testing but proved to have serious visual processing issues, which require quite different testing to detect. Just starting down that road ourselves, but others here have much more experience.
