http://www.fictionpress.com/secure/story/story_tab_guide.php has the guidelines, but they pull from http://www.fictionratings.com. (Sorry, can't make a link to that site for some reason.)

Here's my guidelines:

If your story won't offend a kindergartner's parent, then it's K.

If it will cause a child's parent and hir kindergarten teacher to have a ... 'spirited discussion' about what is or is not appropriate for the classroom, then it's a K+.

If a teenager doesn't find the story boring, then it's at least a T.

But if it causes a teenager's parent and hir teacher to have that spirited discussion, then it's an M.

(And, if it causes a teen to hurriedly shut down the computer / close the window when someone walks in, it's an MA.)

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