My wife and I are in the same age group and sailed with Viking for the first time last year. We did the Arctic Explorer itinerary. Many of our port stops had more active excursion options - sea kayaking, RIB rides, horseback riding, ATV adventures, etc. There were a handful of other cruisers about our age, and a few families with their children who appeared to be in their late teens / early 20's, but we were in the minority age-wise. Our itinerary might have had a slightly different demographic as compared to others that may be less active.
I only mention that to answer your specific question about the age of passengers and provide context, but we didn't see it as a negative at all. Our fellow cruisers were, for the most part, very active (as we noted by those that were also on the more "strenuous" excursions with us and observing people around the ship and in town) and also friendly, social folks.
We wouldn't hesitate to book another Viking cruise if we like the itinerary, as we enjoyed the overall Viking experience and time spent with our fellow cruisers -- and in fact have our next cruise booked with Viking this July.