I (late 30s) took my parents (70s) on their 1st cruise to celebrate mom's 70th birthday & their 40th anniversary. I am a "almost 1st timer" myself, since my last cruise was over a decade ago on Carnival. This is our 1st Celebrity experience. I picked this particular cruise because 1) it was the best deal I could find - actually cheaper than Carnival, 2) I thought small ship size and a simple itinerary staggering sea day and port day would be good for elder 1st time cruisers.
Overall: The cruise was perfect for my vacation purpose. I don't think my parents felt overwhelmed or lost. It was not too long or too short. We didn't feel like we had to do all kind of things and rush around. It was relaxing. Services were great everywhere and the MDR food was good, much better than I expected.
The crowd: As expected, it's an older crowd. There are some younger people (late 20s to 30s is my guess), but no one was too crazy/obnoxious that I have encountered (there're one or two super intoxicated ones stumbling around). There weren't many young children either (I don't mind kids but it's also nice to not have to hear baby screaming, crying, or being ran over by strollers). Mood is causal - no one was really dressing up too formally. I do kind of like a formal night but understandably cruiselines are all moving away from it.
Great location for food/drink - it was super easy to go down to Deck 5 Cellar Masters, Deck 4 Select Dining for MDR, and up to Deck 10 for Oceanview Grille.
Room size was enough for 3 adults: mom and I shared the bed while dad used the sofa bed (which we left it out as a bed the entire trip - easy nap spot by the balcony).
Awesome balcony, which was about 3 times bigger than a normal balcony and it was covered. Only thing was it came with chairs only, and they didn't have foot stools.
Bathroom is tiny. I am not a big person and the shower was a little tight for me.
No issue with water pressure or temperature.
Quiet space amongst the suites, even the room door is fancier looking to match the rest of the wood doors.
Key West is a fun place. No tour needed, just do it your own.
When walking is getting too much (and hot and humid), get on the free shuttle provided by the city transit called Duval Loop. It even has A/C!
Nothing much near the port besides the airport, which is great for transportation purpose.
Consider 1-way rental and drop off at one of the few rental companies at the port. Free shuttle to pier. During busy cruise season, you may find yourself a super cheap rental 1-way (ex. mine was $10 with Avis) - just keep checking on the price as your cruise date approaches.