Ok, I'll bite.
One, the games are different genres, each bringing in trappings of the worlds they represent. GTA is rated 18+ because it recreated a realistic world akin to our own in terms of story and content.
*Guns are real, while Glowing swords and Lightning Bolts are not.
*Cocaine is real, and Skooma is not.
*There is fucking in GTA, and there is not in Skyrim. Mild nudity, what the fuck are you on about? is it any worse then what you'd get in a 12+?
*Mods have no relevance on determining the age rating on a game. You could add sex mods to sims, does that make it 18+?
*It's light swearing, as opposed to the usual F-bombs GTA drops.
*There is no gore, barring the decapitations, which you can do in other games of an equivalent age rating.
Since you are 12, why do you even care?