I have heard that Disney IS lenient about age requirements.
But I think it also involves your parents, and your attitude. Naturally, if you seem like a nice person who just wants to be able to use the teen facilities then I doubt they will have a problem giving you a chance. They will likely let you come in and hang around and as long as you get along with people there shouldn't be any problems.
Just keep in mind that they don't have to let you in, (like they would a 17-year old no matter how he behaved) so it will be up to you to act respectfully with the other teens in the group. Otherwise they might use the fact that you are 18 as an excuse to say you can't come in.
By the way - although they can probably quickly look it up in their records, you don't have to volunteer your age, but if they ask don't lie, just be honest and tell them what you have told us - that you have been on other cruise ships and you don't want to get stuck in the 18-20 year old limbo land.