S. triporcatus do change as they mature. The hatchlings are marbled with white and black. The adult will retain the marbling on the head and legs but the carapace will become more brownish.
BUT, it is important to note that these animals along with Kinosternid turtles look VERY different when raised in captivity.
Wild caught animals will often appear dark and muddy with obscure pattern when compared to a nice, captive raised turtle.

Kinsosternon integrum is a Mexican mud turtle but not the same thing as a Staurotypus. You are not likely to ever see K. integrum offered for sale.