BTW, although seventeen year olds can join a service with their parents permission, Congress wrote a law in WWII, IIRC, that they can’t be deployed overseas until they are eighteen.
In our society we do not accord minors the same rights as adults. We restrict their ability to purchase firearms, to view adult material, and to hold public office. We enact laws that make it illegal for minors to have sex with adults, even if it is consensual. We do this is because we recognize that minors, for the most part, do not have the capability to responsibly and intelligently exercise those rights.
We exempt minors from many responsibilities as well. Minors are often sentenced differently than adults or even exempted from punishment entirely (if they are young enough). Courts usually will not enforce a contract against a minor if the minor chooses to repudiate the contract, even if the minor was capable of performing. The reason we give them such light treatment on these responsibilities is the same reason we do not accord them the same rights as adults. We recognize that it would be unfair to hold someone to these social obligations when they are too immature and too inexperienced to know what they really entail.
Of all the "contracts" a person can enter, probably the most serious is an enlistment contract. At a minimum, that decision amounts to signing away most of one's freedom for several years and undergoing changes that will stay with the enlistee for the rest of his life. If he is deployed to a combat zone, he will see humanity at its worse. He may have to make very hard moral choices that will haunt him for the rest of his life. In the worst case scenario, a person's decision to enlist could lead to a gruesome early death.
We allow most 18 year olds to make that decision on their own. And this is before they've done any "growing up" in basic training.
The decision to enlist is a lot more important than the decision to buy a car or to take out a mortgage. If we only enforce those contracts against competent adults, then surely we could only enforce an enlistment contract against a competent adult as well.
Right now, if someone makes the oath of enlistment at age 18, they are legally obligated to fulfill it. In other words, the law says that an 18 year old is competent enough to make what may be the most important decision of their life.
If we say that any random 18 year old is competent enough to be held to an oath of enlistment, with all the serious risks and consequences involved, how can we say he is not competent enough to vote?