If you have kids, do you set a firm marketing-division limit on them (as in, no grown-up books until you're X years old), or do you take it on a case-by-case basis, or do you let them roam the library/bookstore and talk with them about what they choose, or what? If you don't have kids, how do you handle it with gifts to nieces/nephews/young friends, or how do you hope to handle it with future spawn?
My dad gave me Prelude to Foundation when I was 11-nearly-12, because he was so darned excited it was out in the first place. And I had been reading Anne McCaffrey's Dragonsong series, and the last one was shelved in the adult SF/fantasy section at the bookstore there in Kansas. So the store owner showed me where it was. I was absolutely boggled: there was an entire section of the kind of books I liked best. Including more Asimov and more McCaffrey: who knew they had written more? Who knew it all had a name? I was in a daze for a week. Hmm. I may still be in that daze, actually, fourteen years later. I may have arranged my life around that daze. I think it was late June/early July: school was out, but my birthday hadn't come up yet. So it's around my grown-up-book-iversary.
(When I run out of things to celebrate, I mentally dub it the nanoversary of something: X times 10^-9 seconds have passed since whatever it was, marriage or whatever. Then, yay! Time to celebrate. Happy nanoversary!)
I had read a few adult books before that, including some with "adult content" -- Mary Stewart's Merlin series was a recommendation from my mom when I was 9 or 10. But that was the dividing line for me, and it was a fairly sharp one.
One of my friends recently talked about how her older brother took a King novel from her when she was 12 because it was "too old" for her and how grateful she was and how right he was. And I was just stunned, because I'd have been furious if anyone had tried to do that to me. Scott once told me I "didn't need to" read a certain book, in a kind of closed-off tone that I took as "it'll be too scary for you," so I promptly went out and read it. Turns out it was a tonal misunderstanding: the issue was that the book in question, Piers Anthony's Firefly I think it was, sucked. Oops. But still, I got an allergy to being told books were too old for me when I was 5 and my kindergarten teacher tried to keep me from reading The Prince and the Pauper, and I have not yet recovered from it.