My supposition is that most 17 year olds are not mature enough to make decisions that are as important as enlisting in military service. Some may be but most aren't.
First, I believe one has to be 18 to enlist in the military.
Second, if we're allowing employers and colleges to recruit on campus, are you contending that H.S. seniors aren't mature enough to make decisions about where they will go to work or pursue a degree, either?