I don't think your ever to old for sleep overs, it just seems to take on a new name. I'm not far off twice your age and i still stay at my friends, but now we do it with pizza, wine and movie and or beauty treatments, which are known as girlie nights. Speak to your grandparents see how they feel about having you staying and if there ok with it and you and your friend still want to continue; forget the age thing it's just a number after all