An 18 year old can go in any of the adult venues onboard, even the bars. They just
can order alcohol. There is also the 18*21 Club. They meet up the first night of the cruise with a
DCL CM facilitator to meet everyone and work out a plan of what they would be interested in doing during the cruise. The group can be kind of small or even non-existent, depending on how many are interested in it.
Our DS went when he was just turned 19. His group was about 15 people (the cruise was the week before Christmas). They usually would go to the adult show in the Pub and then hang out. They would meet up most days and do stuff - go to the movies, have breakfast or lunch with the officers. The participated in the carved veggie car races. Went swimming at 3 am. We basically only saw him at dinner and most of the evening shows. They were also able to get some of the board games from Vibe (well, the CM got them) and then they could play them. And had several sessions of Wii games.