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Most cruise lines will not let you on board unless you are travelling with someone over 21 and in some cases 25. Sorry.
Try Ocean Village. You are OK as long as you are all over 18. Also the style of cruising may well suit you quite well (ie casual).
I have pasted their terms and conditions re age below for information.
5. A booking may only be made by a Passenger aged 18 or over. All Passengers who, at the time of departure, will be under 18 years of age must be accompanied by a Passenger aged 21 or over who will at all times during the holiday be responsible for their welfare, conduct and behaviour.
Uk companies will generally let you go on your own from 18 years of age. I ts really only the american companies who need you to be 21 for some strange reason.
Island, Thomson and Easy also take bookings from passengers of your ag.