I think there is a point where kids can be "too young" for a cruise--especially one costing as much as this one does. I think you have to take into consideration the child and that isn't easy when you are "pre-planning" this early for a cruise.
I would ask several questions of myself:
1--How reasonable is my child in new environments? Do they listen to reason and can they "go with the flow" so to speak? If they are a high-maintenance child, I think the risk of it becoming unenjoyable for everyone is too great.
2--What are your expectations for your enjoyment of the cruise? Do you want to go see show? Will your child sit through them? Will you have alternate plans for him or both of you if he won't or do you have to miss out on the show?
3--Does a child at that age really benefit from a cruise or would an all inclusive resort do just as well and maybe be less expensive?
4--It is always a persona decision. What am I willing to do and not do in order to make the experience a pleasant one for all involved?
I come from experience. I took my 2 daughters on a carnival cruise (2 1/2 and 5 1/2) because Disney wouldn't take under 3 in the clubs. I hated it, my husband hated it, she hated it. It was the worst vacation we had ever taken and I contribute it all to her age. Trying to accomodate her schedule with our schedule and what we wanted to do. We even took my 16 year old cousin with us to kind of help out with the kids. She became a kid herself---she got very homesick for the first half but finally came around to having some fun the second.
I have since cruised twice on Disney (Youngest daughter was then 5 and 7) and it worked out beautifully. My son was born before our first
Disney cruise and he didn't go on either of them. He is finally joining us on this one at the age of soon-to-be 5 (he turns the 5th day on the boat). I am bracing myself for some small issues. He has sensory issues so he can get overstimulated. But, as I told my dh, we cannot and will not expect him to be in the clubs nearly as much as the girls have in the past. If we are right, then we have the mindset we need to really enjoy ourselves anyway. If we are wrong and we can't get him out---then darn, we were wrong and we are gonna party. LOL!!
What it boils down to is the decision is purely your own to make. You just can't make a decision like that and close your eyes and hope for the best. The more you plan and the more you prepare for any and all situations, the more you get out of the experience. Expect the best but plan for the worst.
DG