Celebrity and Holland have younger crowds in Alaska than their other sailing areas. There will be plenty of other passengers near your age group on those lines in Alaska.
IMHO for a first cruise to Alaska....Glacier Bay is a must. This eliminates Celebrity however.
Look at NCL as well. They have a younger crowd and you would probably get a better value through them. Many experienced cruisers complain about NCL not being up to their standards. NCL is probably a tad below the others in quality and service but IMO it's not much lower. And for a first cruise you won't believe that's possible. You'll love it.
We've sailed NCL, HAL, Celebrity, and RCL and if the itinerary and price were right I wouldn't hesitate to sail NCL again.
You'll be stopping in Ketchikan and Juneau for sure. Most ships stop at Skagway and a few will stop in Sitka instead. We preffered Skagway over Sitka.
2 fun excursions you could look into......In Juneau take a whale watch trip. Orca ent. is very good. In Skagway the White Pass RR is wonderful, expecially if you like history and/or trains.