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.
Ichthyosis vulgaris slows your skin's natural shedding process. This causes chronic, excessive buildup of the protein in the upper layer of the skin (keratin). Symptoms include: Dry, scaly skin. Tile-like, small scales. Scales colored white, dirty gray or brown â with darker-colored scales typically on darker skin.