I agree that young woman is probably the better translation, but i think youth is okay too (although I'd tend to use it for boys or mixed groups for some reason).
In English, we sometimes lose information present in the Spanish.
If I said, "Esta niña tiene una flor", both "This girl has a flower" and "This child has a flower" would be correct answers, although you do lose information in one of them.
I think it is probably more important to understand the meaning of joven in English than it is to get an exact translation.
If you hear the word in Spanish with "esta", you know a female is being talked about and you can decide how to mentally translate it. If you are using the word joven, if the English is young lady/woman/man, lad, youth, etc. (depending on who you are talking about), when you say it in Spanish, the meaning will still be understood.
I hope this makes sense (and I know it was a bit of a long response to something we are generally in agreement about).
There is a discussion about it here. http://forum.wordreference.com/showthread.php?t=2042722
As with most of these questions of grammar and semantics, don't obsess about it. Talk Spanish to Spanish speakers, read Spanish as much as possible and eventually the way you use the language will become second nature. In time, expressions will just sound right and you won't bother to query why.
Ditto. Understanding the whys and wherefores as much as possible helps communication a lot when starting out. Now after 4 years of experience speaking with native Spanish speakers, for a lot of these I find myself saying "It just sounds right this way, or wrong that way" but even still it is good to know the rules, because I still occasionally will want to say something and not be sure, and can use them as a guide in addition to my experience.
Hahaha, actually I would argue that your answer should be accepted, but it sounds a little funny to me, as "young one" is very common in Hiberno-English for "female teenager/ youth/ young woman", but has a certain connotation! Note that it cannot be used (in Hiberno-English at least) to refer to a male. The equivalent would be "young lad".
In a real situation where it would be evident that the comment is being made about a woman than the observation " this young one has black hair" would not be out of place, i.e. incorrect. But then we are talking about a subtlety that a computer would have difficulties with!☺