Toffs And Twelve Year Olds

I won't allow my niece to read Trouble At Toff Towers as she's only twelve and it contains a couple of "grown up" references which I don't think are appropriate at her age. She does love to read though. And write. In fact we've been working on something together for quite a while now...

...three and a half years to be exact. It's a children's story. The thing is, choosing names for our four main characters has involved endless discussion. We have agreed on three. Deciding on their age group was much easier:

Me: How old do you think the characters should be?

Niece: (Without a moment's hesitation) Nine.

Me: Blimey, you sound very sure of that. Any particular reason you think they should be nine?

Niece: (Knowledgeably) Because most characters in books are nine.

Me: (Impressed) And how do you know that? Did someone tell you or is it simply something you have observed?

Niece: I just noticed it myself. Most of the characters in the books I read are nine.

Me: (After careful thought) When you were two how old were the characters in your books? And how old do you think the characters are in grown-ups' books?

Niece: Oh yeah! (Pause for laughter followed by careful consideration followed by a very serious question) Isn't it a bit boring reading about grown-ups?

We agreed our main characters will be nine. We have also recently agreed there will only be three of them as we never could agree on a name for the fourth. This year we're hoping to start actually start writing the story.