According to the California Turtle and Tortoise Club, it is nearly impossible to accurately estimate the age of a tortoise or turtle, unless it was acquired from an early age. The practice of counting growth rings, while helpful with trees, is only partially accurate with smaller tortoises and turtles.
A tortoise or turtle's growth rings do not necessarily represent a year's growth. Within a season, multiple growth rings may form, especially if the animal is in good health. After 10 to 20 years, the appearance of new growth rings ceases almost entirely. Over time the animal's carapace may flatten and lose its sheen.