I've sailed 5 different lines in Alaska and frankly, passengers are similar in age, middle aged and above the majority, but all ages sailing. EVEN Carnival has this demographic- don't expect the party crowds of the Caribbean.
Overall, for me passengers make no difference, I sail Alaska for what is outside the ship, meaning, routing, and ports. HAL offers what I consider to be one of the best round trip Vancouver itineraries with Glacier Bay and Tracy Arm. My opinion only, I never do any one ways without spending at least another week for interior Alaska travel, take advantage of being all the way there.
Plenty of choices to make, consider ports, time in ports, glacier, route, ship naturalist, price. Budget fully for costly excursions, you get only half a trip without them.