We are in our 60's and have been on 5 cruises this year - Carnival, Princess and Holland, which are all owned by Carnival now. Here is just my opinion.
Longer cruises always have less children and young adults because of who can afford the cruise. And all 3 of them are great, it just depends on You and your wants.
Carnival has better desserts, come on chocolate lava cake every night, Yes! Grin Comedy Club is a big plus. We don't hang at the pool usually because it can get rowdy but find other places to sit and enjoy the ocean. Furnishings are a little less grand than Princess or Holland.
Princess, I like because they do have so many activities to choose from, music venues, games etc, even if you don't participate, watching is fun also. They seem to use all their venues well. I have notice that since Carnival bought them that the food quality and service are not as exceptional as it used to be but is still good.
Holland, rolls up the sidewalks at 9 pm, or so it seems, but they have the best, I mean the best service of all three. And if you like classical music, their Lincoln Center is unique to just that line.