If ever you're 18 years old, indeed it going to be difficult for you to get a job onboard a Cruise Ship. I've been working on Cruise Ships since 1990. The youngest crewmember that I've met was 20 yrs old. But, a very young male applicant with very limited experience ... if ever you make it in, you might end up taking the lowest positions onboard - Galley Utility ; Housekeeping Utility / Steward; etc. Needless to say, these positions can be very difficult and exhausting. Bear in mind, normal working hours would be 12 - 16 hrs / day ... and its 7 days a week. But, if there's any consolation ... promotion is fast onboard coz promotion is based on merit (performance) and rarely by "who you know". Also, if you're really determined or bent on achieving your goals, then I see no reason why you can't succeed. Good Luck, Mate!
Foodstore and others are real experts... but we had a busboy when we sailed on a Celebrity ship who was EXCELLENT. It was his very first voyage and he was just 17. He said that they said you needed to be 18, but he got on. He made up for our crappy waiter.
2 years later we cruised Celebrity again and talked to someone who was friends with him and found out that he had advanced not only to Waiter, but to the Chief Waiter- the only one who was certified to serve the officers.
What country are you from, Tim? Maybe you could get a two year degree in Computer Systems and go on as a Systems Manager?