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This site and others indicate you can't sex a turtle until it is 4".

A juvenile has the appearance of a female: short front nails, cloaca that doesn't extend past the shell, a short tail and other indicators. Our turtle is just slightly over 3", maybe 3 1/8". It still has the appearance of a female. I'm assuming that if you can tell at 4", then there has to be some beginnings of sexual dimorphism before that.

With the understanding that you can't be certain until past 4" or more, at what point do you begin to see longer nails or the cloaca of a male begin to extend? Are there hints at 3 1/2"? At 3 3/4"?

Any input appreciated. I've seen lots of info on sexing, but little on the transformation of juvenile to adult.

Yeap. You can't CONFIRM its sex until its at least 4 inches. But noticeable sexual dimorphism primarily starts after the turtle is 3.5 inches... with the claws and tails starting to get noticeably longer for males...

The way i would prefer to check is the position of the cloaca on the turtles tail... females have their's tucked within the edge of its hind scutes and males cloaca having it extend noticeably beyond the edge. (Note: this is provided the turtle is at least 3 inches whereby that should be a subtle difference between tail lengths bet sexes)

It's possible at 3.5", the only way to tell early is if a male starts to show the characteristics. There are also late bloomers, but that doesn't happen very often.