On his Wikipedia page, in the 1966 - 1975 section it says it took a lot of persuasion to get him to use the wheelchair. This suggests that he wasn't in dire need of it at that point. His first child was in 1967, the last in 1979.
6. Extremely high ammonia levels in the water can damage the outer layer of skin and cause shedding. RES are tolerant of high ammonia levels while map turtles are not, so this problem is more likely to occur with Graptemys species.
A turtle's shell is comprised of about 60 bones; the outside is layered in scutes which are plate-like scales similar to the keratin composition of fingernails. They protect the bones and cartilage of the shell underneath. Turtles do shed pieces of their shell like many other reptiles--but unlike snakes, they aren't shedding their skin.