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.
2. Taking people with limited mobility - my mother finds walking difficult but coped well, particularly when Viking decided after day one to introduce a 'leisurely paced' tour group. There are no lifts on the boat however so to get between the dining room, the lounge or sundeck you need to be able to manage flights of stairs. Actually, the exercise probably did her good! The staff were always very helpful and Viking clearly knows its market and has thought about how to meet the needs of older people.
3. Being organised - you do need to plan ahead and be in the right place at the right time - tours tended to leave the boat at 8.30am, dinner and lunch were served at fixed times. We coped fine with this - we're like that anyway - but we know some people who would have hated being so organised and not getting their lie-in on holiday. Looking back, I think one of the most relaxing thing for me was being freed from almost any sort of decision making beyond what to eat. With a busy professional job and a family to organise, my life is usually full of decisions to be negotiated so this was a real break for me!
4. Keeping in touch with the outside world - there is an OK Internet connection but it's not great - it took me an hour to download The Times. We took a wireless phone dongle to supplement the ship's connection.
We will certainly look at cruising again with Viking.