I emailed Disney about this same issue, as my sons at time of travel will be 17, 15, and 13. The 13 year old cannot imagine not hanging out with his brothers. Here was their response:
Thank you for choosing a Disney Cruise Line vacation!
I would be happy to assist with the teen area on the Disney Fantasy.
Vibe is an indoor/outdoor teens-only lounge for Guests 14 - 17 years of
age. The operating hours for Vibe are usually from 9:00am to midnight.
Hours are subject to change without notice.
Please check with the Counselors on board, to see if they can make an
exception.
...
To insure that only teens enjoy Vibe, a special coded Key to the World
card must to presented to access the lounge. Registration is not
required for teens participating in Vibe activities.
I hope this information has been helpful.
We look forward to sailing with you!