I did this cruise in January of this year with a woman I met through Cruisemates; she had numerous dietary issues and ate only fruit, nuts, and vegetables, and not all of them. We were on a Princess ship and they accommodated her quite well in the main dining room at both lunch and dinner. She took care of herself at breakfast in the cafeteria.
Holland America and Celebrity cruiselines also do this CA-HI-CA itinerary; you should check with a travel agent. If you're not picky about cabin type or location you might be able to get a much better price if you're flexible about which line you travel on. All three are comparable for food and activities; Princess has a problem in that its main performance theater is MUCH too small; people start arriving an hour before in order to get a seat.
You are not likely to run into many twenty-somethings on a 15 day cruise; younger people tend to gravitate to shorter cruises, and often to Carnival. That said, there still ought to be at least a few close to your own age.
The weather in Hawaii is delightful year-round.