Rouen is correct...
It was long thought that one simply had to count the growth rings on a turtle to accurately determine its age, like you would on a tree. This is not exactly true for turtles: each 'ring' can indicate a temporary cessation of feeding or even a dramatic change in diet, and not simply a year's worth of growth.
For instance, a wild turtle could conceivably end up with three rings at the end of one growing season, simply due to eating well for a few weeks and then fasting for a few, eating again, fasting...you get the picture.
In captive raised turtles you might see rings caused by a change in diet.
Another thing to bear in mind is that a turtle's shell does not continue to grow in the same way once it reaches its mature size, so, after a certain age you wouldn't be able to really count rings: also the shell can become very smooth over the course of many years, leaving no rings to count.

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