There are some ships you have to be over 23 and 25 to go on, i know under 21s (i am 19) are limited to Costa, Carnival and Royal Caribbean..
You wont get told your ship until you get details about 2/3 days before you go. you may be in training a few weeks, you may be in there up to six weeks. girls i was with that were hairdressers were still there at 5 weeks..
You get 1.5 days off per week (not all same day) and it depends on the ship your privileges..
Big ships have between 5 - 20 spa staff, smaller ships have between 3 - 10