Does the age of your MC correspond directly with the age the book is aimed at? For instance, if you are writing a book for young adults, should your MC be the age that your book is aimed at?

Have you ever written a book for adults, and yet used a child as your MC? I was thinking about To Kill a Mockingbird the other day. It's a novel for adults, and the book contains subjects such as rape, racism and incest, yet the MCs are children.

Just something I was musing over the other day after I saw a copy of To Kill a Mockingbird in a bookshop.

I don't know if it's a question of "should". Personally, I try to write my books for all audiences, but if I'm writing a YA, I do try to keep my MCs around the young adult age. The book I'm trying to publish now is geared toward teenagers and young 20s, though anyone could read it. The MC is 14 and turns 15 in the story, but she deals with things that are usually considered beyond her years (no spoilers).
I also wrote a YA novel (not that I'm even considering it for publication anywhere near the near future) with a 30-year-old MC. I guess it doesn't really matter, as long as the writing is understandable and relateable for your target audience.

I think they can be any age as long as it is written in a way that suits the audience. Some of the most interesting stories to read are about old people yet not written for old people. Perhaps there is a certain way the elderly can specifically relate to it, but they don't have to be the target audience by default.

Aye. I like to write from a younger viewpoint, for older audiences. I guess I don't like writing age out of my experience, so I write for my age while making my MC younger.... *shrugs*