We travelled on Viking Sun from Basel to Amsterdam (well, we didn't quite make Amsterdam, but that's another story) in August 2012 with my parents who are in their eighties and my 16 year old daughter. We all had a great time and I agree with all the other reviews on here that tell you how wonderful the food, cabins, excursions, fellow travellers etc are. We were extremely comfortable, well fed and well entertained and made some great new friends.
So I thought I'd just mention here a few practical points that may be helpful for others considering such a cruise:
1. Taking a teenager - most of our fellow travellers were over 50 and my daughter plus two other girls probably in their early teens were the only youngsters on the boat. Another couple told us that they had asked to bring their 11 year old daughter but had been told that this wasn't allowed. My daughter is very happy with adult company, quite happy to sit and read a book and is also interested in history and culture so she was very content with the cruise (as long as she could regularly reconnect with friends at home via the Internet). If your teenager wouldn't last five minutes in this environment (like my son) then probably best not to inflict it on them and them on everyone else.