Speaking from experience of the army of 14yrs, I would personally reccomend you do some sort of apprenticeship before you even think of joining the defence, I joined when I was 16 & 9mths with signed parental permission.
You need to live some of your life before you decide to join any of the forces. Ya gotta spend time out with friends and working and partying before you join.
Don't jump into something like that until you have least lived some of your life, and your only 15 I wouldn't go worrying about that type of thing til you finish high school, most defence jobs want you to have all passes in yr 10, 11, & 12 oh and now you have to be 18 to join...