JohnM:
Some thoughts from this cruiser (completed #16 in March, #17 and #18 are already booked) and former travel agent.
I personally favor Holland America because they make no real secret of the fact that they cater to Seniors! Now, please do not see a bunch of grey-hairs pushing their walkers along the deck! But, like Villagers, see a group of, shall we say, "seasoned" individuals enjoying the fruits of their labors and being treated respectfully by the crew!
My second choice is Princess.
I frankly stay away from those lines who appeal to the younger generations (say, our children!). My principal reason: I am NOT into Hairy-Chest Contests around the Lido Pool. I am NOT into loud, raucous disco-style night clubs. If you are, then I'd say look elsewhere than HAL or Princess!
As to how to book, that's an increasingly difficult question to answer. Back in the day when I was travel agent (more than 10 years ago, now) there was so very much less interaction on line. The good travel agents were sometimes considered as True Gurus! Nowadays, you must do much research on your own or expect to be forever confused.
I agree that the cruise critic site is helpful, but also recommend cruiselinefans.com. This is set up and uses the same system as TOTV, and has participants who are just as friendly as they are here!
SWR