Reaching adulthood

When one actually does reach an adulthood.
Watching a five year old in the movie called City of God proclaiming to be an adult because he snorts coke, and then watching twelve year old salesman at a local fair to behave like one 40 year old salesman I know made me think, what does constitute adulthood? Officially it is 18 years because it provides certainty that a human brain has fully developed... But brain, not the user.