Telling the age of your yellow belly can really only be an approximation unless you know when it was born. Assuming you don’t know there are a few things you can check. You can check its shell rigidity, length and width, count the rings of a scute, check its sexual maturity, and lastly if it is no longer living do a skeletochronology test. To see how you can do these steps, have a look at our full guide on telling a turtle's age.