However recent studies have shown that the portion of the brain responsible for decision making isn't fully developed until about the age of 21. At about 21 we begin using an entirely different system for making decisions that is more capable of avoiding bad decisions. Prior to the change the brain uses a system based mostly on pro v.s. con which tends to favor immediate gratification vs long term consequences.
That's just a correlation. It's entirely possible that people don't start making better decisions until 21 because that typically is the age at which people are seen as adults, and people (generally, in my assessment) realize they need to bone up a bit and assume their adulthood. It can also be observed that married people tend to be more responsible than single people (at least, I have).
Children/teens are, in our society, encouraged to make pro vs. con decisions and to not seriously consider their futures. It's all been taken care of, afterall - they've got until after college to screw around, and up until that point, pretty much everything has been determined for them. There's little variance.
Two hundred years ago, it not only happened, but it was commonplace for people to be seen as, and to behave as, adults at ages as young as 14 and 16 because they were brought up right, and weren't put through the god-aweful public schooling systems where mediocrity is the point of excellence (both socially and academically). I was home schooled for a large part of my pre-18 years, and it is my experience that there is no reason why a person can not be "college educated" (knowledge and comprehension equivilance) by the time they're 18, behaving like adults well before that time, as a norm (fairly large groups of home schoolers). And not just the "smart" ones, either - there were quite a few people who would likely test below the national average who were as mature and as knowledgeable as most 21-year-olds at the age of 17 or so.