Please explain why you say that. Keep in mind this is in context of adult material being present in the library.
Because at that point, a normal 12-year old is pretty much mature enough to handle sexual content in written word without taking any damage of it. I know I was, I know other kids around me were, and I make no claim whatsoever to be specifically mature for my age, neither then nor now.
Heck, by the age of 12, kids should have learned about sex in school (of course, many schools are rather uptight about this, but what can you do?). They certainly do around here. By suddenly saying that you can't have the possibility to even read about it is making a bigger deal out of it than it really is. It's just sex. It's not like violence or murders, is it?
What is absurd is that you would have a parent accompanying every child in a library. It is a much simpler proposition that adult material be in an adult section that is clearly marked off limits to minors. Then only one adult (a librarian) is necessary to keep an eye out for any and all minors.
No, I don't want an adult accompagnying -every- child in the library, that is a strawman. But, to avoid more misunderstandings:
Kids that needs to be accompagnied -everywhere else- in public should also be accompagnied in the library. And as long as there isn't a specific appointment going on (like reading hour), -then- that's the job of the parent. Period.
Kids that are allowed autonomous movement in the enviroment around the library (for example in malls). should also be allowed to do so in the library. Of course, many parents have different views on when this age is; and it'll partially also be decided by the enviroment you're in (kids in small towns are generally "let loose" much faster than kids in big cities). Kids that have been taught about sex in school should be considered able to handle sex in literature.
As for an adult section, that is pretty much impractical, because frankly, a hell of a lot of literature not specifically written for a younger audience will usually have -some- sex in it. I started reading ordinary novels at the age of nine or ten, and more often than not, acts of sex were happening. An "adult" section would be completely arbitrarily decided, based on which of the few books would be too nasty, and it still wouldn't do much, because there's still a vast amount of books that a bunch of morally hysterical parents would shout loudly about (mostly thanks to rumours about how nasty the book is rather than having read the books themselves). It would never be enough, and it wouldn't help.
There is already a real children's section, and a section for all the other literature in a library. At least any library I've seen. That is a good enough separation in my view.