We took our first Disney Magic cruise with just the three of us --- me, DH and DD (13 at the time). BIG MISTAKE! We took her to Common Grounds when we boarded and introduced her to the CM there, but she wouldn't have any part of it. She was at that age where she just didn't make friends easily by herself. We (DH & me) couldn't go to the clubs, or anything that was "over 18" without leaving her alone. We managed to go to the comedy club (when they had that back in the beginning of the cruises) and left her in the stateroom watching TV for about an hour. Felt guilty and came back. Thankfully, it was before the 7 day cruises and we were only gone 4 days.
The second cruise (last Nov - Eastern), we took along a friend. Because our DD was older (15) and she had a good friend along, it made all the difference! WE HARDLY SAW THEM! Actually, they did their thing, we did ours, and we met up for breakfast (sometimes) and dinner and sometimes the show, but for the most part, we all had the best of both worlds. Adult time, teen time and family time. They are both very mature. We did not do the walkie-talkie thing. They pretty much were told that they were to stay with the teen club and their activities and, yes, they do have activities that go until 2AM! We would even go to bed around Midnight and they would be in around 2AM. We'd sometimes leave them sleeping in the morning, and catch up with them later. We almost always bumped into them during the day and, of course, on days when we were in port, we were together and they were absolutely not allowed to do any offship activities/excursions without us.
I have to stress that you need to be very sure of whom you take along with you, though. You want to have someone who your child is going to get along with and not grow tired of easily. Our daughter had some friends along in the past (at WDW) and after about the 3rd day, was wishing we were back home again cause they were around each other so much. Make sure they can agree on most things and not have someone too aggressive OR to passive. My DD hates when her friend can't make up her own mind ("I don't know, what are YOU going to have? I don't know, what do YOU want to do?)
Oh, we also are DVC members, so the first time we took along DD's friend, we just used points. But, for our Western next May, we are using points for DH and me and paying for our DD and her friend. We only ask that she bring along her own spending money for things she wants. And we pay for any shore excursions.
You also will need to have her birth certificate and a NOTARIZED letter of permission from her parent(s).
Good luck and have a wonderful trip!