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.
The body continually sheds dead skin cells as new cells are formed. In most cases, this is a gradual process that goes unnoticed. However, this process can speed up, producing excessive amounts of dead skin cells, which causes dandruff. It's not always clear why this happens, but possible causes include seborrhoeic dermatitis (see below), a common skin condition that causes oily skin.