I was on the Magic 7/18-7/26, and from what I saw club O2 was the problem. I have three teenagers, my oldest went to Club O2 the first night and didn't go back. During an ice breaker they went around the room stating what they liked to, she said the most common answer was "play video games and smoke weed" and staff said nothing. It's as illegal as alcohol in this age group and I don't think they would have allowed them to all say they like to get drunk, but this answer was apparently acceptable. It wasn't her crowd so she hung out with us and relaxed reading on the balcony.
One mother told us her teenage daughter was left at her cabin door passed out but doesn't remember what she took. Someone knocked on the cabin door then left her in the hall. There was a fight at club O2, security was called and shut it down. However the teens just gathered in stairways and ran around playing manhunt and knocking on doors, basically being a nuisance so it was reopened. I will say security was everywhere on the ship. We saw them intervene when a parent and grandparent were just screaming at a small child, and again when there was a problem in the hot tubs. They sat outside club O2 a lot.
My other teens found friends at circle C and had a great time with them. They made bracelets and had lots of ice cream and enjoyed the freedom of roaming the ship with their new friends. They said many of the kids knew each other and have been on many cruises and the staff knew the kids, so Magic isn't just full of new cruisers. We aren't the type to be in the nightclub, but honestly besides the family screaming at the child, all the adults seemed pretty chill and we didn't have any problems. This wasn't my favorite cruise (it was our fourth Carnival cruise), I found the entertainment pretty boring (the shows were like good karaoke with a theme, just singing with really no dancing or story), the comedians were just OK, and there wasn't much to do besides trivia (the pools were PACKED) but I loved relaxing on the balcony.
Some other thoughts: portions are small in the dining room, after a couple days many in our group were ordering multiple apps or main dishes. I thought it was a nice way to try things but harder on the staff. Food was fine, some dishes were great, only the lasagna was inedible, but the melting chocolate cake was very soupy every night. We had anytime dining and never had to wait more than a few minutes for our large party of 12. Also it's been said before, but Carnival needs another late night food option, lines for pizza were so long after 11PM and that's were I saw tempers start getting heated. We just paid for it through room service, the pizza wasn't as good but my kids LOVED the donut sandwich. Our steward was wonderful and worked so hard, but only having service once a day meant the couch bed in one room was always set up as a bed instead of converting back and forth to a couch. We enjoyed the spa, I love those heated loungers!