what are some cruise lines that you dont have to be 21 to travel on?

my friends and i are going on a cruise as a graduation present for ourselves.. just to spend some time with the girls before we all head off to college... but we have a problem.. most (if not all) cruise lines state we have to be 21 to board without a parent or guardian 25 years of age or older.. So do you guys have any suggestions or know of any lines that dont have to be 21? thanks :)

Cunard
Disney
Radisson
Windjammer