I have to say that I have experience here. My lucky baby has taken two cruises and he's not even 9 months old. (Took me 34 years to go on one!) We took him when he was "just" legally old enough (they have to be at least 12 weeks). It was a little bit easier at 3 months because he slept a lot and didn't mind being going everywhere. We just got off the Wonder last week and he is 8 months old. This time, since he is crawling around he wanted a little more freedom. I broke down and let him crawl--yicky!!---around a little on the floor in some places and of course in the state room. I do carry him in a sling though which I certainly wouldn't have been able to do without. That way my hands are free as he sits on my hip and I can deal with my other kids, or food, etc. easier. Plus if he falls asleep he just sleeps in the sling and we don't have to change our plans to go back to the stateroom. And it sure beats trying to push a bulky stroller around. The servers were excellent about getting him crackers, etc. at mealtime. It was also easy for us since he is still breastfed I didn't have to worry about bringing food onboard for him. And with all the healthy choices he just ate some things off our plates. During the shows, particularly if it was somewhat dark he would just nurse to sleep in the sling and we could enjoy the entertainment. We never did use Flounders. I wasn't really sure I wanted him chewing on all the toys in there anyway and in close contact with other kids. Plus I'm too cheap I guess.
We hope to go back next May when he will be 21 months. I hope he will like it much better then when he can interact more with the characters and enjoy the shows.
You will have a great time regardless of their age. I think it just makes it different each time you go. Have lots of fun!!!!