I've heard the argument of it being 25 to account for brain development. Unfortunately, brain development is not maturity (at least, not given the number of fortysomethings I don't trust not to blow their own brains out with a nerf gun).
17-18 is for all intents and purposes the age of divergence in this country, where we move from high school to something else. Graduation is the closest thing we have to an OroP mentioned above.
Shit, although my teacher friends will probably blow fuses at the thought, I'd even be down for lowering the age of it all to 17 or 16. Let the early fuckups happen when they're still at least part-time in a nationally structured environment.
Ideally, we'd have parents guiding their progeny through this in stages.
The problem with relying on parents is that, for it to work, you need parents that are There, Willing, and Able. And nowhere near as many kids as you think have the complete set.
And governments make shitty parental substitutes.
And while adulthood isn't a binary concept socially, it absolutely is a binary concept legally.
But where it becomes chickenshit is where we're more than willing to consider 18 year olds adults when it suits our needs (no longer educated on the public dime, a draftable military force) but balk when the concept of them using adult vices comes into play.
And even then, as a society we're not above takes backsies when we feel like it. A fifteen year old commits a heinous enough crime and "try them as an adult" comes in. As if the horse hasn't already left the barn there. It's societally selfish bullshit.
I've led memorial services for people I couldn't buy a beer for. That's bullshit.
I absolutely get everything about the idea of graded responsibility.
But there's a huge fucking gap between "good idea," and "needs to be a law."