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.
Snakes, Lizards and Chelonians. Â©1996, 2002 Melissa Kaplan. All animals, at regular intervals, shed their skin. Some, like humans, do it relatively unobtrusively in a sort of ongoing process (we shed 1.5 million skin cells every hour with a new skin surface every 28 days or so). Others do it quickly and cleanly, like the one-piece sheds produced by snakes and tarantulas.