Ageing your turtle
It is not possible to determine the exact age of a turtle, unless you know its birthday.
You cannot determine a turtle's age by the rings on the cutes. A turtle may grow several rings in a good year, when it is young, and it is growing fast, and no rings at all in a bad year or when it is fully grown.
You can estimate a turtle's age in the following way. What you are really doing is a bit better than guessing.
1. Find out what the adult size for the turtle would be.
2. Measure your turtle.
3. Find out what the maximum age limit for the turtle would be.
5. Subtract some years, if the turtle was raised in captivity, because captive turtles grow faster.
6. Look at the cutes. Has the turtle grown recently? A mature turtle grows slower and won't show much new growth.
7. Has the turtle bred? To do that, it must be mature. Water turtles become sexually mature around 5-8 years of age, depending on species and environment. (For tortoises it varies and can be as late as 20 years.)
8. For omnivorous species: Does the turtle eat/prefer vegetables? Older water turtles eat more vegetables. (But that is not always true. I have a mature male Reeves who eats no veggies, and a juvenile female Reeves who loves veggies.)
9. For Sliders and Reeves, the shell gets darker as the turtle gets older. Dark shells are mature individuals.
10. Disposition: Adult female sliders can be pretty aggressive.
11. Know the gender of your turtle. Females usually grow bigger than males, and a female will often be larger than a male at a given age.
12. Quality of shell. A mature turtle that grows slowly usually doesn't outgrow scratches, pits, or chipped off pieces quickly, so older turtles, especially in the wild, have more "stressed" shells.
Good Luck! And I hope you will find out the age on your turtle.