The conventional wisdom seems to be that it takes a long while for the body to mature. Brain scans seem to prove you have to be at least 25 to achieve good judgment.
From personal experience, I might say it takes longer.
But on the other hand, since I'm contrarian, maybe not. Brain scans also seem to show that the brain is like a muscle, different types of experience leave their mark. For example, London taxi drivers have to study for 3 years to learn all the streets and pass the exam. Scans show the area of their brain devoted to spatial relationships grows over the 3 years. So, maybe we shouldn't assume that the brain scans devoted to maturity simply show the natural development of the body with the logical conclusion that we shouldn't expect mature judgment from 21 year olds. Maybe the scans reflect the progressive infantilization of Americans, the fact that children/adolescents/young adults aren't exposed to situations that test their judgment. We don't have 8-year olds responsible for herding their family's cows. We have (I'd guess) fewer teenagers working jobs and more studying. We have more students in college and post-graduate study. (You learn many things in college, but not necessarily good judgment).l