No method exists for accurately determining the age of a turtle. Some species provide clues via the rings on their shells, but many species, such as soft-shell turtles and types of sea turtles, have no rings to consult. Based on physical appearance, there isn't an accurate way to age sea turtle species — some can live more than 100 years. Consequently, you can only make educated guesses about a turtle's age unless you know the approximate date the turtle hatched.
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Turtle growth rings
Many turtles, especially tortoises and box turtles, possess distinct rings on their plastrons — the part of their shells that protect their undersides. However, unlike trees, turtles do not necessarily produce their rings annually. They may produce extra rings in times of abundant food or may fail to produce rings in years in which they were injured, ill, or unable to find enough food. Usually, food availability and scarcity correspond to warm and cold months of the year respectively, so one ring is approximated as one year of growth. But this isn't an exact metric.
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Additionally, as turtles drag their plastrons on all types of abrasive surfaces, the rings often erode and become indistinguishable by the time a turtle reaches about 20 years of age. In many species, the rings are difficult to identify, creating other problems with this strategy.
Signs of turtle age
You can draw a few tentative conclusions about the general age of some turtles by examining their shells' condition and color. For example, when considering wild-caught individuals, older turtles will show more evidence of wear and tear. Their plastrons will be smooth, their carapaces may have several dents or chips, and they may have scars on their legs or heads.
While such clues do not indicate exact age, they allow you to make the reasonable conclusion that a specimen is an adult, which is to say 10 to 20 years of age or more. Captive-raised turtles are unlikely to have sustained the shell damage that their wild counterparts have, making shell damage an unhelpful criterion. Additionally, some turtles tend to darken with increasing age, while others lose distinctive patterning.
Age of turtle bones
While not a technique available to the average turtle keeper, some scientists have begun examining the bones of sea turtle species — a discipline known as skeletochronology — to determine their ages. Some bones are thought to produce annual rings, so by analyzing them and counting these rings, scientists should be able to determine their ages. The process involves staining a sliced cross section of the bone and counting skeletal growth marks. Clearly, this is done after the turtle died. However, bone histology is not a straightforward process and has limitations for aging.
Lifespan of a turtle
One of the reasons that scientists are interested in determining turtles' ages is because turtles live such long lives. While box turtles and slider turtles may reach 50 years of age or so, some of the giant tortoises live much longer lives. Scientists suspect that the world's most famous tortoise, a Galapagos tortoise named Lonesome George, was approximately 100 years old at the time of his death. One turtle — a Seychelles tortoise named Jonathan, who currently lives as a pet on the island of Saint Helena — is thought to be 189 years of age. He hatched in the early 1830s and has been cared for by humans since the 1880s.