Hey! I'm 15 too and I just got off of a Princess cruise yesterday morning, but I have also been on 4 Carnival, 1 Royal Caribbean, and 1 Disney. The last Carnival one I went on was when I was 14 last spring break. I wore jeans a couple of times in the dining room (just on Carnival), and didn't feel overdressed or underdressed at all. People on Carnival are much more casually dressed than the people on Princess. I wore skinny dark blue jeans with nice baby doll tops and stuff like that. For shoes I think I wore wedges, but only because I felt like it. I think even flip flops would have been alright. Jeans are totally fine for Carnival for a 15 year old girl, as long as they aren't the big flared and ripped up kind. Don't wear them with tank tops though, because those, along with shorts, are not allowed in the regular dining room. Sun dresses are also great to wear in the dining room.