I'm not sure that there is any true age you have to be in order to talk with a recruiter. Many will talk with high school teenagers. Many recruiting stations have a program where those who think they are interested in the military can come to the recruiting station at least once a week and work out with others in preparation for the physical fitness tests and basic training. You cannot, however, join a military branch until you are 17 years of age (with parental written consent) or 18 (without parental consent). If you are thinking about joining the military this is a good way to get information. I do, however, suggest you have a parent talk with the recruiter as well....whether it is with you at the initial meeting or afterwards. It is always good to have others hear what you do in case you want to talk anything over. They may also think of questions you don't. I would also pay attention to how firm of a commitment they can make you. For example, if you join the army because you want to train as a helicopter pilot and you are told they will TRY to place you in that job, it isn't the same as a guarantee they will.
I will also put in a plug for the Boys Scouts of America. The highest rank a scout can achieve is Eagle. When applying for the military, it is my understanding that anyone who is an Eagle Scout coming into the military automatically gets bumped one rank up.