I don't know about NCL, but Carnival is very strict with their age groups. You can ask, but, when we sailed the Elation, there were no exceptions. My kids were older and it actually worked out for them. They both met friends their own age.
Star........go ahead and ask; some posters have said they won't bump up, but might move down------that would mean your older son would have to move down. If they won't budge, put on a positive face in front of the kids, even though you will be disappointed. And remember, there will be other "almost 6's" in with your 5 year old. The older kids in the age group tend to make friends and they'll have a great time. It also gives your older son a chance to be with the older kids and make his own friends. Either way, whatever happens, be positive and it will work out. The counselors are great and make sure everyone has a good time. Have a GREAT cruise!