I too would endorse Vancouver, unless the price-difference is so great that it's impossible to ignore. Departing from Seattle would mean missing the magnificent Inside Passage. There's also the possibility of flying into Seattle and taking ground transportation to Vancouver.
I think that Holland America's reputation as being "for older people" is undeserved. No, you won't find fellow passengers doing beer-can-building contests during the wee hours - but it's also not as though the whole ship shuts down after 9 pm. We took a cruise 15 years ago on the last season of the old Rotterdam - our sons were 9 and 10 and had a wonderful time.
Do some research before automatically booking ship-excursions for the ports-of-call. Some, you can easily do on your own at a fraction of the cost. Some examples: the Raptor Rehabilitation Center in Sitka - an easy 20 minute walk from the harbor. Or, the horse-drawn carriage town tour in Ketchikan and the hop-on-hop-off town trolley in Juneau.
May will likely be rather chilly, but it's a good value - and really, no one books Alaska and expects to sit out on the pool deck all day. However, if you really want inland travel (and I certainly recommend it) you might want to look into a bit further into the season.