"She is a girl."
Translation:Она - девочка.
"Девочка" in Russian refers not just to a female person, but to a young girl (younger than 12 or so), and can be contrasted with "девушка" (a teen or young adult female, or "girlfriend" in some contexts) and "женщина" (a woman). So, you might see a sentence like this used for disambiguation, but it would generally have some additional context, and/or come in response to a question.
I see what you two mean. I guess my issue here should be with the English sentence. I understand the Russian translation makes perfect sense, and that is in line with zirkul's comment. She is a "young" girl, or She is an "old" girl, etc. makes sense. In English, you wouldn't say "she is a girl" - you would have some type of adjective like "young" or "old" or "pretty," but those things that are automatically understood in the single Russian words for "young lady" or "woman," etc.
Девочка can have another meaning: a virgin. Does "a girl " have such connotation? I.e. "She is a girl. She is not yet a woman". Or it does not make sense?
By the way, there is a song "I'm not a girl, not yet a woman" by Britney Spears. It may not be about virginity, but about teenager's psychology. Anyway, if a person says "I'm not a girl", then someone might be telling about her "She is a girl". Wrong? :)
What about transsexuals? I.e. there are three persons: a girl, her father, and some stranger. The girl says, "I'm a boy!", but her father cries, "She is a girl!".
Forget about the transsexuals :). Someone says, "Oh, your baby-boy is so beautiful." You correct them, "She is a girl." I did that several times already in Russian. How do you say such things in English?