I was reading Read Roger this afternoon and his post discussed an article in Publisher's Weekly entitled "When YA Might Not be OK." In this article the librarian said that she often gets confident readers of 10 or 11 who are wanting to read up. They want things like Twilight, or Pretties, or Gossip Girl. And she doesn't feel comfortable giving them to them, so she attempts to stear them to more suitable material. Roger said it, "gives [him] the willies." And it sort of did me too.
When a kid asks for a title specifically I do my best to locate that title. I don't feel its up to me to judge whether or not they should be reading it. I might think they aren't old enough, but I certainly wouldn't say anything. If luck has it and it isn't available (and usually even if it is) I suggest titles that I think would suit them better. As in, "and while you're at it why don't you try these titles too? I think you'll like them."
My best example is a voracious 11 year old reader that was one of my most regular patrons a few years back. She ran up to me one day and told me she had just read Candy by Kevin Brooks and had absolutely LOVED it. Never would I have thought to recommend that title to her, but after that (and with her father's permission) I started recommending all sorts of titles. After all, why not? I certainly read up at her age.
Essentially, what my reference interviews boil down to is I ask what books they have enjoyed in the past and try to recommend titles based on that. After all, the whole point is to encourage kids to read, not to stifle their interest. Nothing says, "don't bother ever coming to ask me for help again because I am not really interesting in helping you," than saying no you can't read that book because I don't think you're not old enough for it. I'm not saying don't use your judgement, but an 11 year old wanting to read Twilight? Sure bring it on.