How Old Is My Turtle?

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.

1) 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.)
2) Measure your turtle or tortoise.
3) 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.)
4) Guess based on size.
5) Subtract some years if the turtle was raised in captivity, because
   captive turtles grow much faster because their diet is usually richer.
6) 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. 
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 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.)
9) 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.
10) 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
    environment.)
11) 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.
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!

How Old Can Turtles and Tortoises Get?
=======================================

Like with all animals, different species and individuals within a 
species can reach varying ages. There really are not any definite and
exact studies on longevity of Chelonians in the wild. There  is 
some information available on captive Chelonians when people
kept/keep records.

In general, water turtles are estimated to reach about 30-40 years of age. 
Box turtles can get over a 100 years old but seem to range about 50-80 years.
Large tortoises have been known to get over 300 years old; a good assumption
for a tortoise is 50-100 years.
Sea turtle life span appears to be similar to humans.

Animals kept in captivity and under proper conditions (!!!)
live considerably longer than animals in the wild, because they
get veterinary treatment for diseases and parasites, and they
are protected from weather extremes and predators.

In any case, the most important thing to remember about how long
turtles and tortoises live is that they live for a very long time.
If you decide to keep Chelonians as pets, be willing and prepared
to provide for them for a long, long time; really, pretty much for
the whole of your life.