I keep seeing the phrase “young girl” used in all sorts of contexts. It almost always bugs me, because it can refer to such a wide range of ages, and it's rarely clear from context which age it means.
It can mean a two-year-old girl. It can mean a ten-year-old girl. It can mean a fifteen-year-old girl. In some contexts, it can even mean a twenty-two-year-old woman.
The word “young” is relative, and the word “girl” is used in a lot of different ways. But the word “girl” generally implies young, so adding “young” to it adds a weird kind of emphasis without adding clarity.
So next time you find yourself about to use the phrase “young girl,” think about whether there's a clearer way to communicate the age that you have in mind.
(Yes, yes, it may be perfectly in keeping with a character voice to use this phrase. I'm not saying there's never a good reason to use the phrase. I'm saying that most of the time, the phrase is ambiguous enough that if your goal is clear communication, you're probably better off using a different phrase.)
(Wrote this in January 2012, but didn't post it. Am currently (April 2015) reading a piece of fanfic that frequently refers to Hermione as a “young girl,” which reminded me of this peeve, and I figured it was time to post it.)
(On a side note: I only very rarely see “young boy” used the same way. The fic in question does use “young boy” the same way, and that bugs me too, but not as much as “young girl” does.)