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 few keepers get concerned that their tortoises skin is peeling. This is seen as white, flaky wisps of skin detaching from especially around the neck and legs. It has been suggested that this is a sign of low humidity in the enclosure. While I am pleased that people are starting to pay such attention to humidity levels, it is not the problem in this case.