Mai Ku Tiki-
I am under the impression that the main reason you are considering holding off on the cruise is so that you can all participate in the same activities together, not so much because of the shorts issue. Is that correct?
If so, I understand your dilemma. While there will be plenty of things you WILL be able to do together, there are other activities that he would be excluded from. Depending on what you are looking for in this vacation, that could be no big deal, or it could really take away from the experience.
We have cruised twice and plan on booking again in the future. I also have two boys, though they are younger than yours. My oldest will be 13 in December, and my youngest just turned 11. We decided to wait until my youngest turns 13 to go on our next cruise. I think it would be more fun for them if they could both participate in the teen activities together. I really can't see my older son going to Common Grounds by himself, and my younger son never cared much for the Oceaneers Lab on either of our past cruises.
I know that this isn't exactly the same as your situation, but I understand why you would want to wait. If you think that this will be your only cruise as a family, and you think you will all get more out of it if both boys are with you for most of the things you choose to do, then by all means wait another year. But if you think that they would still have fun spending part of the time together, and part of the time doing their own thing, then go ahead as planned and have a great time!
BTW, how far away is your younger son from 18? Would you really have to wait a whole year, or would it just be a matter of months?