I think Princess is the most "young" of the 3 lines though you will see few 20 somethings on Princess, I am guess average age is 35-40 on Princess.
HAL in the Caribbean is more ages 35 and up, but has many smokers, HAL has a less stringent anti smoking policy compared to Princes and Celebrity. HAL works for younger smokers in the Caribbean.
As far as Alaska, even if HAL is an older crowd, you need to realize Alaska attracts an older crowd, so if you smoke, HAL may be best for you.
Equal opportunity cruiser since 1998-4 Carnival, 4 RCI, 3 HAL, 3 Princess, 5 Celebrity, 3 NCL, 1 Disney
I have now achieved-
ONE HUNDRED DAYS ON A CRUISE SHIP!!!!!!!!!!
146 days total