As to the above post, 12 year olds aren't allowed in The Stack unless they are within 30 days of turning 13.DD (a veteran of 10 cruises on the Magic dating from the maiden voyage) celebrated her 13th birthday on the Magic. She absolutely loved The Stack. It is not as cozy as Common Grounds was, but the chairs can all be moved to make a dance floor. There are 2 good sized rooms, so different activities can be occuring at the same time.
The teen programming is limited, but some of the activities (like the Gotcha game) were a big hit. There is a list of activities that can be done at any time--the teens just have to ask the counsellor for the supplies. In general, she enjoyed the activities, and also enjoyed just hanging with the kids she met. My DD doesn't like rap/rock,etc. music (she's an accomplished classical musician), but didn't have any trouble with the music in The Stack. Her only "funny" comment was that the counsellors were "teen wanna-be"s.
Maybe we were lucky--since our cruises are during the school year, there are not a lot of school aged kids on board. There was a relatively small, young teen group on our cruise, but they all seemed to have a good time. DD has stayed in touch electronically with several.