I don't know about the Pirate Cruise, but my DD just did the Sassagoula Adventure Cruise, which is a similar cruise out of POR.
She just turned 5 at WDW, and was easily the youngest here. I'd guess the other kids to be 7 and up. It worked out fine, because they seem to make it so that all ages can participate and all of the kids looked like they had a great time. In fact, the older kids seemed more into it than my DD was, lol.