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I got three RES last July, and ever since I've wondered what gender these are. One of the online sites I visited said that males have long claws
but that only happens until they are six (if my memory doesn't lie).

If you have any information regarding this, please post it.

"If something is worth doing, do it right." James P. LeBon

They can not be sexed until they are over 4 inches. And you are correct on males having very long front nails. But if they are smaller than 4 inches, you'll have to learn patience like the rest of us hatchling owners. Their gender characteristics don't show up until they are around this length many times, so it is wise to wait to know for sure.

Try using the search function at the top and you will come up with several posts on this topic, some include pics to help you tell the difference.

Thanks for this information. But on average about when does this happen (when the turtles exeed 4 inches).

That depends on a number of factors, including diet, warmth and even the grow rate particular to the turtle. With many captive turtles, given the high protein diet they tend to receive, I think the four inches is reached at about a year (some keepers turtles have even been larger than that at that age). This is not, necessarily good, however--you want your turtle to grow slowly, but steadily, and if your turtle doesn't have a shell length of 4 inches at a year (but isn't really small), if s/he's healthy otherwise, that's good.

Some turtles will show secondary sex characteristics at less then 4 inches. My RES was less than 4 inches (around 3.5), but with his long nails and shape of the tail, I was very sure he was a boy.