You cannot determine a Chelonian's age by the rings on the scutes.
It may grow several rings in a good year when it is young
and growing fast, and no rings at all in a bad year or when
it is fully grown.
You can estimate a Chelonian's age in the following way. What you are
really doing is a bit better than guessing.
- Find out what the adult size for the animal would be. (You need to know
the species for that, which is a good idea anyway, since different species
have vastly different requirments.)
- Measure your turtle or tortoise.
- Find out what the maximum age limit for the turtle or tortise would be.
(There are no exact numbers available, but water turtles generally live
to about 30-40 years, box turtles and tortoises to about 50-100 (and
longer in some documented cases)
- Guess based on size.
- Subtract some years if the turtle was raised in captivity, because
captive turtles grow much faster because their diet is usually richer.
- Look at the scutes. Has the turtle grown recently? You'll see growth
patterns that look new. A mature turtle grows slower and won't show much new growth.
- 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.)
- For water turtles: 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.)
- For red-eared sliders and Reeves turtles, the shell gets darker as the turtle gets older.
Dark shells are mature individuals. In many turtles and tortoises, the shell markings are
livelier and more pronounced in younger animals.
- In younger animals, the growth patterns on the scutes are distinct, and the
scutes show more texture. In older animals, the shell gets smoother and may
be completely smooth in an old animal. (But, this varies with species and
- Know the gender of your turtle. Females usually grow bigger than males,
and a female will often be larger than a male at any given age.
- 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.