If the line advertises that it has facilities for families look at the age groups it caters for. Be sure to check there are set aside facilities for the teens as they like their own space. If that is provided then the teens will get together, make new friends and have a great time. No cruise line wants bored teenagers rampaging around the ship upsetting other passengers !!! P&O, Ocean Village and RCI seem to get the most positive comments in reviews. I have used P&O and Ocean Village and found them v good. Ocean Village even provide shore excursions for teens. Much appreciated by both parents and teens. They also look after younger children while parents are on excursions. In other words dont just take agents say so (they may be selling on commission), check carefully in the brochure, look at the deck plans to see how much space has been allocated, and ring the line direct with any in depth questions. My teens did NOT want to go on a cruise. Suffice to say, a couple cried and one had a stiff upper lip when time came to leave the ship. They are still in contact with friends they made.
The Voyager & Freedom Class Royal Caribbean Ships have got be the teenagers dream cruise. Ice skating, roller bladeing, rock climbing, surf boarding (not Voyager), Johnny Rockets, plus all the usual video arcades etc. No contest with the others!