Going melanistic in sliders is much like male humans going gray - it can happen at any age, but one this dark is likely a very old turtle. I've seen a pet male slider one that was completely dark at 15 years old. I think the rugosa (the wavy portions of the shell) is more a sign of old age than the coloration.

Katrina
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1.2 Eastern Muds - Fred, Ethel, Edith
0.1 Iguana - Tiffel
0.1 Bearded Dragon - Foster
Foster turtles: More than I'd like the husband to know about.