Amsterdam to Europe - British Isles & Western Grand Circle Cruise Reviews

We were asked by retired friends of ours to join them on a river cruise on Grand Circle Travel's "River Rhapsody" for a cruise up the Rhine and Mosel rivers. We have been on numerous large ship cruises but never on a river ship. We had a couple of major concerns; "would there be enough to do on such a small ship?" and "we will probably be the youngest people on this ship". We are in our early 50's ...