Well, I have only been on one cruise(Royal) and I learned a lot. Formal night attire: A sports jacket and a nice pair of slacks is fine for a man or you can wear a suit or tux, you'll see it all. For women, a nice pant suit or slacks and shirt, dress(doesn't have to be the Hollywood kind). You'll see a bit of everything.
The ships has something for everyone. Its doesn't matter what your age group is. I am 36 and had a ball. You will not feel out of place or bored.
Where are you going? Have fun!