Did you book this directly with HAL or through an agent? Regardless, the booking shouldn't have gone through is the ages were not in line with the rules.
But beyond getting all your money back, you have no options with HAL. It is obvious HAL is not going to let you board the ship. All I can say is look at booking with another cruise line and making some adjustments in your plans.
As an aside, I have to say that a 20-day cruise on Holland America is a curious choice for people 23 and 20 years old. HAL typically attracts an older clientele anyway, and any cruise that is 3 weeks long will be mostly "older" people regardless of the cruise line.