The turtle's cloaca is the key to figuring out it's gender. The male's cloaca is further down the tail then the female's. Look at the underside of it's tail, and estimate the length between the cloaca and the back of the shell. Alligator snappers don't become sexually mature until around 10-15 years of age, so it may be difficult to tell until then.
Age is the harder of the two to determine. Turtle shells in general add ring annuli - little concentric rings in the shape of their scutes - as they grow. However, one ring doesn't necesarrily equal one year's growth. They just represent growth spurts. If the turtle was wild-caught and then became captive, this will add to the confusion, because it's growth and eating habits probably changed then.
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