We actually went years ago when my kids were still small (the oldest is 12 now) but we were given free tix and we were there mostly during the daytime hours first, and only stayed for the first part of the "Scarowinds" (I think they now charge separate admission for the night part). I didn't think it was bad at all and I was pretty protective of my babies. They weren't nearly as phased by things as I always thought they would be at their young ages! And as a strong Christian, the blood and gore part of Halloween bothers me, but if you know some of the background of the holiday (people dressing up scary to be passed by the 'evil spirits' that came looking for souls to possess), you realize that it wasn't originally something that was to glorify evil, but to avoid it. Anyway, enough lesson, back to the question - I definitely think a 12 and a 17 year old would do fine, as long as they are aware that someone might jump out and scare them. I wouldn't worry about them wanting to "go goth" afterward or anything! And as far as PG-13, as far as I recall, it is due to the violence of some of the characters/events that you can choose to go into (Haunted House) more so than inappropriate dress - I don't remember much sexy stuff, if that is what you are worried about.
As much as we hate to let our kid go - they must grow up and be allowed to make their own choices, and we just hope that they do not depart from the way we trained them up to go! :)
Hope they have fun!
A.