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.
Reason for Scute Shedding. Shedding of the scutes is a routine process during which they peel and flake off and are replaced by new ones. This occurs as part of the turtle's normal growth, as the shell expands with the rest of the growing body.