I can tell you more or less what my kids have done and when, BUT, nowadays, I am more of a mind that there are some things that need to be left until children are a bit older because many of the themes are not really suitable and also because their wee brains miss so much because they don't have the level of understanding required. Various kids have read all of these at one time or another, so my informed opinion is:
Hitchhikers 12/13 or so
Skellig - was set text for DS1 in 2nd year, so about 12
BITSP - not really a children's book, tbh,
Clockwork Orange - definitely an adult book, I read it in last year of secondary, as did DS1
Artemis Fowl - 8 and up (best books for boys I have EVER read, DS1 is 20 and in 3rd year of an MSci Hons and still reads these)
Robinson Crusoe - 12 or so
My Family - teenage (grown up themes)
Invisible Man - 15 or 16 at the earliest, or it just escapes you.
Anne Frank - if you want to read the whole thing, slightly older teenager (boys often find it embarrassing and there are clear sexual themes etc.)
Iron Man - perhaps about 8 or 9, but you get much more from it if you are older
Stig - 9 or so.
War Horse - early teenage
Tom's Midnight Garden - set text for 11 year olds here
Uncle Montague - 12 or so, DS2 used to love Chris priestly
Dark MAterials - 1st one from around 10/11 (DS1 read when about 8 but missed most of the significant stuff until reread much later) other 2 are definitely teenage books, especially the last one which is really almost an adult book.
Holes 10 and up, you get more from this one if you are a bit older, as well, and know a bit about civil rights and whatnot.
Catcher - teenage (was a set text for higher english when I was at school so 16 year olds) younger kids cannot relate to it at all.
Art spiegelmann - see, I would put these with the manga stuff, so although reading level is about 10 or so, thematically they require a lot more maturity. I would say definitely teenage books, maybe even older teenagers of 16 or so.
Frankenstein - Ds1 was about 15 when he read this, as was DS2, none of the others have tried it yet. It is heavy going as are things like Dracula, Jekyll and Hide...
HMAHD - all 5 of my kids loved this at about 6 or 7 years
Adrian Mole - mid teens, younger kids just don't get it.
Talking Turkeys - I would reckon at least 12, you get much, much more out of this if you are a bit older. Benjamin Zephaniah was on the reading list for DS1's 2nd last year at school as part of the literature analysis stuff.
Like seeker, I think that there are so many really good, age appropriate book for children that there is no need to push them on to things that they are not really ready or able to appreciate. Much better to widen their scope within kids books and save the other stuff for later.