We went on a southbound (Seward to Vancouver) on HAL's Ryndham about four years ago and LOVED it! Alaska is an incredible place. We loved it so much we're going back this summer on HAL's Maasdam in July. The previous cruise was before we were
DVC members, but this time we're trading out and using points. You'll hear that it's not the best use of them, you can rent them out and pay cash for less, etc. But for us, we had the points and it was worth the convenience to just use points. Plus, we paid a small fee to upgrade and will have a verandah mini suite - perfect for watching glaciers and wildlife.
I would say definitely go for it. Take a good camera and LOTS of film (or better yet, a digital one). You'll want some good binoculars for wildlife viewing as well. Layered clothing is a good idea as you never know what the weather will be like.
Most imporantly, check into land excursions before you get to the ship. You can book independently if you want or through the ship either prior to or on board. You'll pay less if you book on your own, but they don't always guarantee timing and if you miss the boat returning, you're on your own. There is incredible wildlife viewing, glacier excursions, fishing expeditions, etc. in every port there. You can find lots of info on the Web and HAL will send you info as well.
We had a mix of ages on our boat, but mostly older folks (this was after school had started). Didn't see any kids that I remember, and we don't have any so I can't speak to any programs they may have. HAL is a great line, very elegant and classy without being too stuffy. The staff is friendly and accomodating.