Dan, I agree with a lot of things you say but I have to disagree about anyone being an "adult" at age 17; I don't think that's possible.
I went into the military at 18, barely out of high school as was also true of virtually all the youths I went in with. I, and the others, had the mental capacity to learn the things our schools taught us. Then, as a group, we had the skills to responsibly do serious work in the field but few - if any - of us had the life experience to vote wisely.
In general. our whole life experience had been living with and being fed by our parents, our every need had been provided by our parents.
So, the military could quickly prepare us to fight vicious battles but they could not make adults out of us by simply training us to operate flamethrowers or aim cannon. Truth is, we were still just 18 year old boys with deadly skills and weapons, we hadn't yet experienced the adult responsibilities that could foster reasoned political votes.
I believe 21 is the youngest reasonable voting age even for those in arms. (And maybe 25 for those with no honorable DD214! ???)