Hello Donna, Unfortunatly for Carnival is 4 Months. I personally don't agree with it. But that is the policy as I have been told.
I think it should be 2 years if they are not occupying a 3rd berth that could be sold. But rules are rules and we all have to go by them.
As f-mattox said the discounts are for 3rd and 4th guests regardless of age.