K: 5+. Good, clean fun.
K+: 9+ Deals with slightly more adult situations. May contain gag humor and very mild violence.
T: 13+ Deals with even more adult situations. Mild bad language, mild violence, implied sexual activity, kissing.
M: 16+ Completely adult situations. Contains bad language, violence, nudity, and/or implied sexual activity, but no actual sex.
MA: 18+ Contains very graphic violence or sex. If there is sex in the fic, it is automatically a MA rating.