The last time I checked to book a young client (19 year old), which was about 4 months ago, NCL did allow 18 year olds to book a cabin by themselves. Princess was also allowing this, however, I think they may have changed their policies.
As far as children having their own cabin, it has been my experience, as a TA and a traveler, that as long as a parent or legal guardian is in a cabin within close proximity to the childrens cabin, the cabins can be reserved under the children's names. I have done this numberous times for clients and for myself twice. I'm not fond of making reservations in one name an then switching cabins once onboard the ship. It creates the possibility for some real hassles as far as the Sign and Sail card go. If it were really necessary to satisfy the cruiseline's policy, I would do it, but prefer to just book things as they are really going to be.