What do you want to see? How much activity do you want to do?
I went on a cruise a few years ago with my folks; granted, I was just about the youngest one on the ship (early 30's), BUT, there were so many activities on board as well as off, it didn't bother me... 'cept when I tried to get involved in the country dancing, and the staff virtually ignored me in order to dance with the 'older' single ladies... fine.
Anyhoo, it was a Holland America cruise to/around Alaska, hence a lot of off-ship activities. And it was gorgeous. I would personally NOT recommend Holland America, but I would HIGHLY recommend a cruise to Alaska.
My parents are FAR from 'mundane sedate 50+' - in fact, as I type this, they're just finishing a cruise with 5 days in Madrid, after seeing Greece, Egypt, Malta, and I'm not sure what else (they really wanted to see Libya, but thanks to the US Gov't's poor manners, we're not welcome there right now
).