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.
Kidney disease may result in loosening of the scutes and loss of parts of the shell surface. Whitish discoloration of the skin may be normal skin that has failed to shed or may indicate infection. Fungal disease may present as white or gray patches on the skin or shell and may be associated with pits in the shell. The shell should be hard, like bone.