You are correct! Look at the carapace–top part– of the turtle’s shell. Do you see all the plates, like puzzle pieces that fit together? Each one of these is called a scute. Each year of life adds a new growth ring to EACH scute on the back of its shell, just like a tree gets a new ring each year. This isn’t a fool proof method for determining age though as the rings get closer and closer together and packed more tightly into a small space, making it difficult to be exact. Box turtles have a very long life, living in the wild for 30-40 years! There is a box turtle on record that lived over 100 years! Imagine fitting 30+ rings onto each of those small scutes! It is safe to say though, based on the rings on these two turtles, that one turtle is older than another.
The turtle below has no ridges on its scutes and the one above has many. Thus, the top turtle is older than the bottom turtle. Can you count the rings and give an age estimate? (Without the turtle shell in front of you, this becomes really challenging!)