I don't know if its advice or not ... but I would say that 3 or 4 is the earliest that a child should be brought to a Disney park. Any younger than that, and they're not only not going to remember it, but not know enough going on to enjoy it. At the age of 3 or 4, they won't particularly remember it when they are older, but they are going to have a blast. Pictures help. I am sure that tidbit of advice will translate well into at least a Disney cruise, if not cruising in general.
And I say that with almost 5 years of experience working at Disney parks and resorts. I have seen it all. :-p
Disney Dream - Dec 2012 4n Bahamas
Carnival Imagination - Dec 2011 4n Western
Carnival Imagination - Nov 2010 4n Western
Costa Atlantica - Dec 2009 7n Eastern
My opinions are mine and mine alone. They do not represent the opinions of The Walt Disney Company nor any of its subsidiaries.