We took our DD2 (2 years and 10 months) and not potty trained on the June 4th Mexico cruise. I worried that she would be disappointed to not be in the bigger pool with her older brother (DS9), since she wants to do everything he does. But amazingly enough she was quite content in the splash area ear of the Mickey pool. I think she thought it was awesome that she was climbing in and out by herself, jumping and splashing in the water. Also, when she would get out we would get her something to munch on or a popsicles or something. Scoops has popsicles - but you have to ask for them. She LOVED it.
She LOVED the nursery; they did a cute handprint craft keepsake for us. If I had realized how much she was going to love the nursery - we would have taken her more often.
Even though she wasn't old enough for the kids club, she was allowed to go in and play in the cool play area (with the bigger kids) as long as one of us was with her.
She was entranced during the Golden Mickey's, Twice Charmed, and Disney Dreams shows. I worried about whether she would sit through them, but she was totally engrossed in the show.
She had a blast on our beach excursions and totally got into pirate night!

She was dressed up and saying 'AAARRRR' to everyone she saw! She danced and danced and danced at the parties.
Last but not least - the characters, the characters, the characters! Even just watching them from a distance made her happy and the look on her face when Minnie left a photo shoot and stopped in the hall to give her a BIG hug was priceless.
So my advice is: if you want to wait, then wait. But, if you don't want to wait, then do it and DON'T worry, your little one will have a BALL! Disney knows how to make everyone have fun! All the people were SO wonderful, my little one chatted with our stateroom host, servers, and concierge staff as if they were old friends.