I am not sure what Amy means but don't worry about it...
In fact 18 is a rough age for cruises because many of them do not allow you to drink unless you are 21 - and there are no activities for 18-21 year olds specifically.
I suggest you choose to hang out with the 17 year olds - you are old enough that one year does not make a big difference.
Disney is very kids-oriented, so either you hang out with the teens (there should be a good number of them) or you will be very limited to a small number of adult only activities - mosty of which include drinking.
I also suggest you focus most on what you want to see in Europe - that will be the highlight of your cruise more so than the ship experience.
Try to enjoy whatever activities you can find - the teens have lots of cool things like Karaoke, recording studio, DJ booth, private pool, etc. Other than that Disney has great shows and movies. I hope you have fun - thanks for posting.
You might want to post this on the teens board as well.