Ahoy, Tasha!
When determining where tweens and teens are to be placed for the
Edge Tween Club or
Vibe Teen Club, it is generally done by age. There are always exceptions, and if you have a child you feel should be placed differently, you can always go and speak with the counselors in the respective club. Depending on the number of children aboard the ship, they may move your child based on your concern.
The one hard and fast rule is that once a child turns 18, no matter if they are in high school or not, they are considered an adult and will NOT be allowed into Vibe.
In my experience, when we have had our tween or teen with us and want them to move clubs, it has not been an issue. This is not a guarantee, and at especially busy times might not be possible.
I hope this helps with planning for your cruise! Please visit the Disney Parks Moms Panel with more questions as they arise.
Happy Cruising, Tracy V.