It really has nothing to do with the brain's development, but more so with society and parental upbringing because today's society/parenting shelter their kids and try to shield them from the real world, rather than teaching them reality. (I know, what's more real than student's getting shot and killed at school). Many young adults are naïve and when some start to get a taste of the real world they have been sheltered from (rejection, disappointment, failures, etc) , they don't know how to handle it and snap. Good decision making comes with upbringing, and continues thru life as you make mistakes, learn from those mistakes, and mature. Heck, we have many middle aged people who still make bad decisions, some are also responsible for mass murders..
Edit: however, according to google: a person's brain doesn't fully develop fully until age 25. But isn't development also contingent of what information is introduced to them and what level one exercises (uses) their brain?