If you cruise within 30 days of your birthday, you are allowed into the next age group--for instance, you must be 12 years and 11 months. So, turning 13 during the cruise gets you into The Stack. DD turned 13 on one of our cruises. THe only problem was that we couldn't register for the Wild Side excursion because the computer kept insisting that she was 12, even though she was 13 by that day. No problem--the CM got a supervisor on the phone who took care of it for us. THe CM was great--she insisted that she knew there was a way to make it happen, but she couldn't get the computer to cooperate.
They try to be strict about 13 (or 12 11/12) because one of the hits of The Stack is that "little kids" can't be there. In reality, the 8-12s "invade the stack" when it isn't open for teens. It is doubtful that they would allow the 12 year old. Remember, there is a huge age gap between 13 and 17. It is even greater if they allow a 12 year old who isn't almost 13. However, your 13 year old and her 12 year old friend can hang out in the Quest area as well as the many public areas of the ship.