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.
Lizards have dry, scaly skin that does not grow with their bodies. Instead, most lizards shed, or molt, their old skin in large flakes to make way for the new skin growth underneath. The exception to this is with the alligator lizard, which may shed its skin in one piece, like a snake.