Political rant - age of majority

Was just having a discussion with someone about two topics you'd think would have nothing in common: the drinking age and the Affordable Care Act (ACA). The crux of the discussion was not the merits of either or - rather, it was the distinct disdain with which we treat adults in the U.S. depending on context.

What I mean by that is that we generally treat 18 as the age of majority. In most jurisdictions, under most circumstances, your 18th birthday marks you as an adult. You can vote, courtesy of the 26th Amendment to the U.S. Constitution. You can enlist in the armed forces without your parents' consent - and if you are male, you are required to sign up with Selective Service(!). You can get married without your parents' consent. You can buy cigarettes and other tobacco products. You can enter into contracts legally. You can be called to serve on a jury. You can buy a firearm (though handgun purchases often require higher ages depending on jurisdiction). You are tried as an adult regardless of the crime. And so on.

Yet these same men and women cannot buy liquor in any state in the U.S. for three more years. Again: you can get married and join the Marines at 18, but you can't have a beer on either occasion.

And here's where the ACA came in. The ACA allows you to keep your "children" on your insurance plan until they reach 26. Even if they are married. Even if they no longer live with you. Even if they are no longer a dependent. Even if they are no longer students. The stated reasons for this - job turbulence and/or adequate coverage during college - are unimpressive; after all, these "children" can't stay on their parents' car insurance or tax returns just because they don't have a job or adequate income. But the issue to me is that it continues the infantilization of adults, allowing them to delay adulthood or regress from it in a way that seems to me to be very unhealthy to society over time.

So: what is to be done? While I am normally very much a small-l libertarian, I do think the time has come for a Constitutional amendment to define the age of majority. Insofar as the 26th Amendment already guarantees the right to vote to be 18, 18 seems like the most appropriate number to pick.

I know the immediate response to my recommendation: "Not all 18-year-olds are mature enough to do X". I know this because it was the reason for raising the drinking age in the 80s and 90s. It's the reason cited for handgun purchase restrictions. And some states support higher ages of majority in certain contexts. But that "child" is mature enough to marry, vote, and serve in the military? Really?

Fine: let's repeal the 26th Amendment and set a new age of majority of....what? 20? 21? 25? 30? I am old enough that I don't actually care what the age is. But to me if you are adult enough to do the things I listed at the start of this rant, you are an adult. If you're not an adult for the purposes of drinking, you shouldn't be an adult for the purpose of marrying or smoking or voting.

What do you think? If you want to engage, don't give me a simplistic answer like "drinking is different" or "that's the way it's always been" or "it ain't broken so let's not fix it." Tell me why adults can't be adults in every facet of their lives at the same point - and make clear why other restrictions are somehow more adult than marriage, contracts, and military service.