In the wild, aquatic turtles are known to survive from 40 to 70 years while certain terrestrial species (including the eastern box turtle, Terrapene carolina) may live 100 years or more.

The largest Illinois turtle is the alligator snapping turtle, Macrochelys temminckii. In some locations it may grow to a shell length of 30 inches (77 cm) and a weight of more than 250 pounds (112.5 kilograms). The largest Illinois specimen on record weighed about 160 pounds.

The smallest Illinois turtle is the spotted turtle, Clemmys guttata. Its greatest recorded shell length in Illinois is 4.7 inches (12 cm).

Many species of turtles have temperature-dependent sex determination (TSD). The sex of the embryo depends on the temperature within the nest at a critical period. For these species, hatchlings may be all male or all female.

In Illinois, turtle eggs typically require about two months to hatch. Hatchlings of some species, like painted turtles (Chrysemys picta), overwinter in the nest and do not move to water until the next spring.

Most aquatic turtles hibernate underwater, often burying in the bottom muck. All but the softshells have thin-walled sacks attached to the cloaca that absorb oxygen from the water during winter.

Yellow mud turtles (Kinosternon flavescens) typically inhabit temporary prairie ponds for two or three months in the spring, then aestivate / hibernate on land throughout the rest of the year.

Softshell turtles have a hard, bony shell. Their name refers both to the leathery skin covering the shell (as opposed to hard scutes) and to the rear portion of the carapace that is tough but flexible.

The flesh of eastern box turtles (Terrapene carolina) may be poisonous at times because their tissues store toxins of poisonous mushrooms that the turtle may eat.

With age, the shell and skin of male sliders (Trachemys scripta) become dark in color. The turtles appear dull brown to black, losing the bright colors *that characterize females, juveniles, and younger adult males.