How to tell the age of a turtle?

I have two turtles so I want to know how to tell a turtle's age.
Answers:
I own two turtles. If they are wild turtles it is almost impossible to tell the age for sure. Domestic ones are a little bit easier but not by much. Domestic turtles will grow faster than their wild counter parts. The best you can do is identify the turtle species and look up their max (normal) size and guess from there. Size is measuring the carapace (top shell) length from front to back. Counting the rings will not give you an accurate number. They grow a ring with each growth spurt, and if it is a good season or young turtle they can have between two and four rings added in just a few months. If it is a bad season or an older turtle they may only add one ring in a whole year. So it really comes down to an educated guess unless you know for sure when the turtle was hatched.

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count the number of rings on his shell! Just kidding, I don't know. ;-)

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By counting the amount of rings on it's scutes a.k.a. it's shell

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Accurate Way: Watch it be born.

Way for breeding purposes, etc: Go by size most of the time rather than age.

Way to satisfy your curiosity: Count the belly scale rings. It is not scientifically accurate, but it works OK for human 'need to know just because' curiosity.

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I own a russian tortoise and a yellow-bellied turtle it is impossable to telll there age to go to your local vet.
GOOD LUCK!!:)