Red-eared sliders are among the most popular aquatic turtle pets on the market. These animals are long-term companions, as well-kept turtles can live in captivity for up to 50 years. That said, it can be difficult to determine the age of your slider if you don't know when he hatched. There are clues, however, that can help you guess the approximate age of your turtle, including the size and color of his carapace, or top shell. Knowing the sex of your turtle is helpful, as females tend to be larger than males and so are sometimes identified as older than they really are.
Observe your turtle's coloring. Red-eared sliders are brightly colored when they are less than six months old; hatchlings' shells are often kelly green with light yellow markings. After age 1, these colors fade to a muddied, olive hue. After age 2, the turtle will become increasingly darker until his tell-tale red "ears" seem to disappear and his shell turns a brownish or ochre shade.
Measure the diameter of your turtle's carapace edge-to-edge. When your animal was hatched, his shell may have been no bigger than your thumbnail. However, by the time he was 4 inches long, which is when he could be legally purchased from a pet store, he likely already reached his second birthday. By the age of 4, a turtle can be anywhere between 6 and 8 inches long. Growth then continues to about 12 to 13 inches, after which it is almost impossible to determine age by size alone.
Examine your turtle's front and back ends. After the age of 2, male red-eared sliders develop elongated claws and tails. The claws are especially prickly and refined between the ages of 2 and 3, and become duller and shorter over time.