We just returned from our first cruise - we went to Alaska and had a very relaxing time.
My spouse and I are in our late 20s-early 30s and though we were part of the small younger crowd, we never felt out of place. Whether we were 30 or 80, we all giggled when the seas got a bit rough and we bumped into one another swearing we hadn't drunk anything yet, though our walk told another story.
From the time we got to the terminal, stood in line, checked in--everything went great! Seattle was a great city, though we only stayed the night prior to the cruise, we saw a little of it. The Mayflower Park Hotel was fantastic and just minutes from the cruise, space needle and about 6 blocks from the Pike Place market.