You can roughly tell how old a box turtle is by counting the rings inside one of the sections on the top shell. There will, roughly, be a ring for each year. However, this will vary for captive box turtles. The ring forms when the box turtle goes without food during the winter while it is hibernating. If the turtle is kept indoors and fed all winter, no ring will form. A box turtle reaches maturity between 4 and 7 years of age. Four years for a Carolina 3 toed box turtle, 7 for a western ornate box turtle. At maturity, a well-developed box turtle will be about the size of a half a medium grapefruit. Generally, they won't get much bigger than this. However, when I was a child I saw an ornate box turtle the size of a dinner plate! It had been driven from its wooded home by a housing development. No telling how old it was. So as you can see, you can't really tell accurately, at least by the folk methods that I know. There may be some scientific way to determine age, but if there is, I don't know it! Here is a link to an article I wrote about box turtles and their care: http://www.associatedcontent.com/article/938979/box_turtle_care_proper_housing_and.html?cat=53 I hope this helps! :)Suzanne