We just did the Northbound itinerary on Radiance with 2 teens. They were sufficiently entertained with rock-climbing, mini-golf, ping-pong, and the arcade. There is a teen center with the usual video game platforms, though ours did not make use of them or the sports court that was popular with others.
Plenty of seniors on the ship as well.
Our Northbound itinerary was a little different than the roundtrip itinerary from Vancouver you mentioned, in that we had an additional stop at Icy Strait Point, where we all loved the ZipRider and the port area. With 1 of our Sea Days including glacier-viewing (Hubbard Glacier), there was only 1 complete Sea Day (aka "Cruising"), and I'm not sure that another Sea Day, as is on the roundtrip Vancouver itinerary, would have been as good for the teens.
Our cruise ended in Seward, from which we took Alaska Railroad to Anchorage, as mentioned in the previous post. Highly worth it, even if just Adventure Class fare on the Railroad.