Depends on what kind of climate you live in. Here in New England they have a growing season so one section of scutes is always bigger. You can count the portions that are bigger and that generally tells you the age. In terms of climate it makes sense since the summer months are plentiful for food so the turtle grows more than in winter when food is scarce.
In places with constant growing seasons and captive bred turtles this falls apart and people generally rely on the count all rings then divide by 2.
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