We were just on the 1/17 Magic cruise and my DD(7) thought the 5-7 group was "babyish". She had done the same activities two years ago when she was 5. Since I had heard about the 30 day rule and she was no where near 30 days from being 8..actually 10 months away from being 8, I never considered her changing groups. However, on the first night, one of the counselors recommended that she move up to the 8-9s. We jumped at the chance and went to check into the 8-9s the following morning. No problem...the only thing they mentioned was that she would not be given the option of signing in and signing out by herself. That was fine with me!!
End of story....she had a fantastic time in the 8-9s. Met lots of friends and loved the activities.
I think they take it on a one-to-one basis. I heard there were over 1000 kids on our cruise but the clubs never seemed crowded.
Good Luck