I have only been on one Princess cruise, on the Grand Princess. On that sailing, anything seemed to go in the dining room. I saw jeans, polo shirts, t-shirts, suits, sports coats, a golf shirt with a clip-on tie, and even a tux here and there. Seemed the crowd dressed more uniformly on Carnival! Formal night had the same mix as any other night, although the guy across from us with the jeans did wear a burgundy dress shirt (no tie or coat)! Kids can look good any night in a button-down shirt--add dockers for smart casual and a tie for formal and you are in like Flynn! Ours (12 at the time) wore a suit for formal, same coat (blue) with khakis for informal, and a polo shirt (same khakis) for casual. Looked like a million bucks, only took two shirts, one suit and a pair of slacks to do a 7-nighter. Rest of the time he was in his shorts and T-shirt!