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.
After they reach the outermost layer of epidermis, they eventually shed. The fibrous protein i.e. keratin is meant to protect the skin and provides water-proof barrier to the skin. The Melanocytes: These are one of the types of cells in epidermis which produce pigment and give color to the skin.