^ They started releasing TES games as 18+ because of nude patches for Oblivion. They were forced to raise the game rating from Teen despite the mods were fanmade.
Skyrim was released as 18+ probably only to avoid that - it has almost no "adult" themes in it, the war between Stormcloaks and Imperials looks like taken from a Disney movie (pillaging? rapes? civilian casualities? nope, only brave men fighting for their homeland). Skimpy armour and killmove mods won't make it adult - you would need to rewrite almost all quests.
Just finished playing Game of Thrones RPG - now this is some serious adult stuff (although technically flawed).
EDIT: I know adult for some people means dirty diapers and unpaid loans, but we're talking about fantasy here