We are at about age 50 but extremely active, and some days wore us out. The excursions were extremely interesting and most had good guides with excellent grasps of history. We also took the optional excursions, some being great, others not so great. We learned so much about Europe, more than I ever did in school. Yes, there were dead times, but when you work 50 hours a week, a few hours of down-time are appreciated. We spent hours on the sun deck, watching beautiful vineyards and ancient cities going by as we got some sun. If the one-man band drove us crazy in the lounge, we'd either go up top or to our room, but this wasn't often. When the ship was docked for the night, you weren't locked-in, you could hit the town and party with locals. Our best meals were in some of the restaurants in the towns.
If you are looking for a party ship, go to the Caribbean. Yes, most guests were over 55, but there were teens with their parents and grandparents as well. We were hardly the youngest aboard. The excursions required brisk walking for long periods, but for those less-able, there were slower groups with less walking. The people at our dinner table ranged from pompous rich people all the way to the chicken farmer we met (he and his wife were our favorites).
The crew was almost annoyingly accommodating. They tried their hardest to make everything right and did a great job. The breakfast buffet got a little old but overall it was okay. Dinners were fabulous, bar service was good. Cabins are tiny, but don't stay there- go out and enjoy your day. You can survive without your computer and Facebook for a few days, you just have to remember how you had fun before the computer ran your life.
Overall we were very impressed and will recommend Viking to our friends and family.