We were on the Spirit for a fantastic Alaskan cruise and we loved every minute of it.
In our opinion, the food was great, the staff extraordinarily helpful and friendly, and the ship was spotless.
We don't smoke and never ran into any problems with other people smoking. Never saw anyone inside the ship smoking except at the casino and certainly not in the hallways.
As for people on the ship...we met lots of nice people (couples, singles, etc.) from all over the U.S. and Canada. While waiting for our excursion bus in Sitka, we started talking to a nice man in his thirties who was traveling with his grandfather on a Princess cruise. We asked them how they were enjoying the cruise. Both said the food was terrible and they described their cruise as the "Almost dead or newly wed" cruise. Boy were we glad we were on our beautiful Carnival ship.
We've never seen the "party" crowd on both of our Carnival cruises. We've seen some fun people having a great time but not crazy people.
As for dressing up for formal night. I've always seen women wearing beautiful dresses and men looking very handsome.
I'm sure Mexican Rivera cruises are less formal then Alaskan cruises which are more formal then Caribbean cruises. Crowds differ in each region but as for ship beauty and staff, the Carnival Spirit was great.
We have a 16 year old and consider ourselves not part of the "younger" crowd and certainly not part of the "older" crowd....we're kind of in the middle.