Our son has been going to Disney since he was 5 months old. Now some days, we only spent a few hours in the park, but he loved looking at all the action going on, and all the color.
Probably depends on the child, and the parent, and your expectations. When we took the
DIsney cruise, we knew it would have compromises, cause pretty much every day with a toddler has compromises. But no place is better set up to deal with small children than Disney.
If I were going to take one
DCL cruise in a lifetime, I would probably wait until DS is 5ish. But since I knew we'd probably be back, and I didn't have to pay anything but port taxes for him, and we got a really great deal, and we live in Fl and didn't have to fly, it made sense for us.
We were also willing to try Flounders, which worked out great. He got play time and his videos, we got alone time.
It just depends on your situation and outlook, I guess.