I'm not sure when you last had a child in the 18-21 group but from our experience on our EBTA last May, it didn't work that way. It seems to depend on the CM hosting the group as well as the make up of the group. The group DD was in did a lot of their planning on the fly. The only thing set up was the meeting place each night and then the group would decide what to do.
They also had quite a few kids who joined later and many times they would go out a look for others their age to come join them.
DCL is strict about the ages, though. One young lady turned 18, 3 days before the cruise ended and she was booted out of the Vibe and sent to the 18-21 group that night.