Considering its Celebrity and touring the Mediterranean, there likely won't be too many teens. Granted, this is just relative to other cruises, for example a spring break to Mexico. You will still meet kids your age, and you will still have a great time.
I don't really know what Michael is referring to when he says the midnight buffet doesn't exist anymore, my last cruise (which was last year) we were at the buffet later than 3am some nights. They didn't really put out special food though, that could be what he was saying. It was usually leftovers from dinner and dessert.