Potential to enlist isn't the same as enlisting.
Considering defending the country isn't the same as doing it.
Thinking about service to country isn't the same as serving it.
Let those who actually chose to defend the country enjoy an additional privilege.
I agree with much of what you said. No matter what, someone who *has* served their country deserves full treatment as an adult. Ideally, that is how people should be able to take on the rights of adulthood; by demonstrating their maturity.
Unfortunately, that's not the system we have. For a variety of reasons, we assign (most) of the rights of adulthood at a certain age. Within this system we can debate what that age should be. But whatever we decide is the proper age, there should be some consistency in how we recognize rights once a person reaches that age.
It is inconsistent, in my opinion, to say that everyone at 18 can make the VERY serious decision to enlist in the military, but that they are incapable of being trusted with the decision to drink a beer.
I think you may underestimate the significance of the *potential* to enlist. When we say Joe Snuffy may enlist, we are saying that he has the capacity to make a very serious decision that will affect his life and the lives of many others. Just before Joe Snuffy signs the dotted line, he hasn't proven anything about himself. As far as we know, he's just like any other 18 year old out there. He may have never worked a day in his life, owned up to his past mistakes, or contributed anything else to society.
But we still believe him to be capable of committing himself to several years of military service. That's putting a lot of confidence in someone who hasn't demonstrated anything about his maturity at that point. Why is he capable of this decision- without having ever proven anything about himself, mind you- but not the decision to buy a beer?
Would you likewise restrict the right of an 18 year old to marry, enter a contract, purchase a firearm, vote, or exercise any other adult right if he had not actually served in the military?