This is why you book with an agent and let them take care of this for you. Most of the time, when you call the cruise line, you're dealing with someone who works in a call center and they are only there to sell you a cruise.
Here's RCCL's policy concerning the ages;
What is Royal Caribbean International's Age Policy - Royal Caribbean International
Basically, if you have someone younger than 21 in the cabin, then you must have someone 21 or older in the same cabin. The only time they will wave this requirement, is when the children are traveling with their parents and their cabin is directly across the hall or adjacent to the parent's cabins.
However, if someone is under 21 but over 18 and not traveling with parents is when the problem arises. Obviously, if they're in a cabin with someone over 21 it's not a problem. If they're over 18, they don't need a notarized statement from their parents giving permission for you to take them out of the country. You only need that when they're under 18. It's that 18 to 21 age range that's the problem area.
Easiest solution; have your agent book one of the adults in each cabin so you satisfy the age limits. Then when you get onboard, you can either swap keys, which I don't suggest as this can create problems because you are constantly re-swapping the keys every time you leave or return to the ship. So what I suggest is to go to the Customer Service desk once you get on the ship and ask them to swap the keys electronically for you so the keys match the person and the cabin they'll be sleeping in. We have clients doing this all the time and it's not a problem.