We went on the Zuiderdam this past July and we are 34 & 40. We met at least 7 other couples in our age bracket. Those are just the ones we spoke to. As well, there were plenty of children, many more than what i would have expected. I enjoyed the Zuiderdam so much I just booked the Oosterdam to Alaska.
I think there is a lot of negetive press about HAL being an "old couple" ship and their new vista class ships are nothing like that. It's certainly not a party boat, but every cruise is what you make of it.
Grand Princess 12/05
HAL Zuiderdam 07/05
Caribbean Princess 08/04