We retired in August and took our first cruise in March. We chose Viking because it's adults-only. As a retired teacher, that sounded fantastic! The food was amazing, the shows were fun, and the excursions were educational. The ship was gorgeous and we quickly found "our" spot to relax in the afternoons. We learned that we liked sea days as much as port days.
If you enjoy a younger demographic, Viking won't be your cup of tea. We are both around 60 years old and we were younger than most of the other cruisers by far. Viking is also pretty pricey, but you probably know that already.
Our only concern is that we took our first cruise on Viking and it will be hard to top that!