Most big modern ship have a full range of options from relaxing on deck - to disco's etc. If you generally want more formal experience (dressing up) try Cunard or Celebrity, for more 'party' (informal) feel (with lots of families) try RCI or NCL. If you want more sedate, on smaller ships, try Fred Olsen.
Hi, there are no age limits on any of the cruises and i am led to believe that most cruises cater for all ages. I am 28 and travelled on the QM2 Southampton to New York in Nov last year. My husband and i met a really great lady from Cardiff who was in her 30's and was travelling alone. Alot of cruise ships are about socializing (especailly at meal times) and i know cunard tend to sit the single passengers next to one another at meal times were possible. The lady met lots of people on board and even got to have a boogie! The nightclub on board the ship was great for everyone and the ''pub'' was also great for socialising. Cunard also have ''single travellers'' meet ups to ensure that lone travellers get the opportunity to participate with activities if you are not brave enough to stand at the bar alone (which i don't think i would be). I can only go off the views of the lady we met and we did end spending alot of time with her, but she certainly had a great time! I hope if you decide to book, you have a great time!