Wev'e only cruised Princess before (twice), so I may be off base for this one, we are headed on our first Carnival, the Destiny, on the 28th of this month.
My son took his first cruise at age 11 is now 13, looks much much older though, passes for Jr/Sr. in High School with both kids and adults. He hasn't been in less than a suit for any formal night dinner yet. On the first cruise he wasn't thrilled with me. . .but on the 2nd he even choose a new shirt and tie. We bought a black suit in JC Penney mens dept for around $100.00, he choose a white shirt for the first formal and a dark burgundy shirt (which was AWESOME with the black suit and looked "modern" but still very dressy) for the 2nd. Both nights he wore a Jerry Garcia tie(which he likes so much he now has two AT HIS request).
We never dress up at home and I think its good for him to learn there is a time and place,and how to dress and behave in that setting. And he actually made sure his suit still fit for this year without me hounding him! He seems to be looking forward to doing it this year. To each there own, I know kids that wouldn't be caught dead in a suit and thats ok too, I don't get offended by that. . . .just thought I'd share my two cents.