i did some reading and apparently it's illegal to show simulated sex with a minor even if the actor in question is not a minor, just the implication of filming fake underage sex is a felony (http://news.cnet.com/Law-on-fake-child-porn-upheld/2100-1023_3-202322.html) which is why i'm guessing they upped her age and changed it from statutory rape to nonconsensual rape...afaik we don't know dany's age in the show ever, do we?

plus for real, she was naked a lot in the first season, they wouldnt have been able to show her scene with the whore, the scene where she changes drogo's positions, stepping out of the fire naked, pretty much the whole empowerment of dany thread would've been out the window if they had her character be the same age she was in the books.