You wouldn't normally be sent to a war zone until you were 18 years old.  There are some exceptions to this but they are very rare.

On the other hand, bear in mind that if you turned 18 while in the forces, and if six months had already passed since you enlisted, then you could be posted to a war zone any time after your 18th birthday and you would not normally have a legal right to leave the forces for some time after that.  For more on this, see the article on leaving the navy/marines.