I'm pretty sure the rules are still the same in regards to underage nudity in films. The Hole only got a 12A (12 and up) rating in the UK, despite having a topless 15 year old in it. Sophie is 17 though, and I don't think it's as much of an issue once you're past the age of consent. Quite a few highly regarded films have nude scenes with actresses aged 16/17 (American Beauty and Romeo + Juliet, off the top of my head), so I don't think it's much of an issue in the US either. I don't know what they plan on doing in regards to scenes with Tyrion and Ramsay, but I think it's safe to say no one's going to jail over it.
You can appear nude at any age with parental consent. Brooke Shields was 12 when she did nude scenes for 'Pretty Baby' and Keira Knightly was 15 when she went topless for 'The Hole'. Sophie is 17, so if she wanted to appear topless, D & D would certainly not be arrested for allowing it, but I'm glad they've chosen not to. I wouldn't be surprised if they used Ros for a fake Sansa plot at this stage. I feel like they've been leading up to it a fair bit, with her having red hair and blue eyes, being from Winterfell, and having a history with both Theon and Jon.