Hi,
We have been on 4 cruises. 1 on Norwegian to Mexico and the other 3 on Royal Caribbean. None of them were super formal, although they do have a formal night when a cocktail dress would be fine. The rest of your clothes can be really casual--shorts, capris, swim suits, etc. Our 3 cruises on RC went to the Bahamas, Key West and Mexico, and St. Croix, St. Thomas, and St. John. My husband and I are 35, so we were late 20's and early 30's when we went and saw a variety of people on each. We took our daughters (10 and 7) on the last one in February and they LOVED it. We booked the same one for next February (the one that goes to Key West and Mexico). Most cruise lines do a great job catering to people of all ages.
My parents love Holland America (especially the Alaskan cruise) and Crystal Cruises. They are more formal and a bit pricier. My brother loves the Norwegian cruise to Bermuda. He went on one in April and the weather was iffy. Might want to go a little later to have better luck with the weather. My husband and I went on a vacation to Bermuda early this year (not a cruise) and loved it!!! It was really clean and we felt safe wondering the island. The people were super friendly, too. Bermuda is a great choice!
The activities are numerous on the cruises and so are the excursions on shore. Food is ALWAYS available and I feel like I gained 10 pounds after the cruise!
A lot of people don't feel like an upgraded room is worth the extra money. If you are never in your room, then this is probably true. My husband and I like to have coffee on our balcony in the morning and a cocktail out there in the evening before dinner.
We enjoy having a little extra space (and I do mean LITTLE---the cabins are super-tiny).
Have fun and let us know what you decide.