Well, for me, it all circles around to responsibility, and at what point does a person become legally and morally responsible for their own actions. Especially those with potentially lethal consequences.
We have an all-volunteer military force - that is no one is forced into military service, in theory. So everyone who is serving in the military is there because they chose to place themselves there. Barring stories of shady recruitment tactics, those people are assumed to be able to make informed decisions, weighing risks, benefits, and their own moral limits - and are assumed to be capable of making those decisions responsibly. So, at 18, the US gov’t assumes that someone is responsible enough to choose to place themselves into situations where the range of outcomes can boil down to kill or be killed.
Which, implies to me, a high degree of presumed responsibility on the part of the people recognized as being able to make that choice.
Every argument that I know for having a drinking age higher than 18 seems to be based on the premise that the gov’t or society cannot trust these same people to make responsible choices when dealing with booze. Usually with drunk driving as being waved as the worst potential decision these people might make. So society as a whole is willing to trust an 18 yo with the decision to risk his or her life (And the lives of others.) in combat, but won’t trust that same 18 yo to be able to choose to not drive while drunk.
I do not see how one can argue that the 19 yo is incapable of being trusted with booze, but can be trusted to know his or her mind enough to accept deadly peril as a job.
It’s not that I think that “If you’re happy to enter a bar you should be happy to fight in Iraq.” But I do think that if you’re trusted to enter a bar, and drink responsibly, you should also be trusted to make a decision about serving in the military, whatever that choice might be. We constrain the choices of minors, or require parental permissions, for many dangerous, or life-defining events. Marriage is one of them, as is enlistment of minors. But, for almost every other aspect of public life, once someone is 18, they’re assumed to be able to make sensible and responsible choices, until they prove to the contrary.
It’s only with booze that this legal and societal assumption is waived.