Welcome to the site.
You have a Brazilian slider? They're quite beautiful...
To measure your turtle accurately, measure the length of the shell (not the diameter) from the back of the neck down the spine to the end of the scutes just above the tail. Do not include the curve of the shell in the measurement. This is called the Straight Carapace Length (SCL).
Unfortunately, you can't accurately tell how old your turtle is by the size of the shell. This is because how fast a turtle grows depends on a number of things--including genetics, diet, general living conditions. A turtle that is fed a lot and grows quickly may be larger (but younger) than one that has not, for example.
The only way to accurately know how old your turtle is, is to know when s/he hatched.
How long have you had your turtle? Has your turtle grown noticeably since you got him? Do you have a picture (I'd love to see him).
Posted: Sat Mar 04, 2006 12:54 pm