There are exceptions, adults may have guardians and are effectively considered minors if they have mental problems for instance, and in some cases, children can be emancipated. But these are usually for extreme cases.
I saw some neuroscience research once that indicated you don't really mature until your early 20s.
That said, in some cultures you are considered an adult at about 14 (mostly hunter gatherer cultures that lack the insane complexity and fundamental weirdness that we have right now).
18 seems a good rough split-the-difference, but equally, in an ideal world it'd be 21. That said, most people wouldn't want to wait that long to do responsible adult activities like drive cars too fast around the place, vote for the people who do the best job of lying, and consume borderline-lethal quantities of alcohol ... :-)
I consider (myself) and most people I know to have been children at 18 and up to several years older than that. But it's not like people wear their "maturity" levels on their forehead, so the justice system has to make an arbitrary cutoff somewhere.
Not saying it's ideal, and individual circumstances have to be considered, but it's the best a crude system can do.