The one RCI cruise I took as a teen with the curfew in effect (it wasn't pre-2005 I believe), it was enforced. And it was enforced strictly. Enough that I didn't cruise RCI until I was 18. Basically what happens is that security asks you to go to your room or escorts you there if you refuse. So yeah, be prepared for room parties. As far as I know - from people who have sailed, Oasis and Allure are strict with the curfew.
As Rebecca said, formal night is your call, but I recommend a suit or tux. Always a good opportunity to look sharp.