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No cruise line will take unaccompanied children - have to have over 21 or on some ships over 25 in the cabin.
Remember 18yr olds are not children in the uk. you have to be at least 21 on American lines to book or even to drink alcohol. The only exception to the alcohol rule outside the USA that I personally know of is Sea Princess as it has always been aimed at UK cruisers. Though in American waters that can change.
If you are 18 I would suggest Ocean Village is the cruise for you (having been on OV1 and Island Star) The nightclub is great and in common with every girl I know - they ALL dress up in the evening especially if they are going clubbing. Research the internet for a good deal and enjoy your cruise. My girls love it !!
I've checked the brochures and both Ocean Village and Island Cruises allow bookings from anyone aged 18 or over but ALL members in the party must be at least 18. So from what you say with regard to ages you should be ok. These two companies operate a very casual approach to cruising (ie no formal dressing up in the evening) which may suit you. They are also not too expensive as cruises go. OV operate larger ships and promote the younger approach to cruising. I now also see that P&O (but not Princess) allow 18 year olds to book. I suggest you get some brochures and/or check out their web sites.
My daughter and her friends went on an Ocean Village med cruise to celebrate passing their A levels, so they were all 18. They had a great time and us parents were so glad they hadn't wanted to go to Ibiza or Ayia Napa. I'm sure you would have a great time, just as they did.