The easiest way to determine the age of a box turtle is to look at his plastron and count the rings on it. The plastron is the ventral, or abdominal, part of the animal's shell.
Each year, a turtle’s plastron grows a new layer, resulting in the appearance of rings, much like a tree's rings, on the creature's shell. The older a turtle gets, the harder it is to use this method to determine his age due to wear and tear on bottom of his shell. This method is highly accurate for turtles younger than 10 years old. Once a turtle reaches 20, the rings are partially worn off. By the time the turtle is 30 or 40, the rings are typically completely worn away. If the plastron rings are not visible, another way to determine a box turtle's age is to count the rings on the carapace (the top of the turtle's shell); however, these also wear away with time making it difficult to determine the age of older turtles with accuracy.