Adolescence, in terms of brain development, happens between the ages of 10 and 19. Recent research shows that span might need to be extended almost to the mid-20s. While many animals give birth to offspring that can walk and seek food immediately after birth, human babies must develop these skills over the years. Adolescence is the same way and appears to take much longer than previously thought.
Because there is still physical and mental growth going on, even until legal adulthood at age 18, it's hard to say when, regarding chronological age, a person is truly an adult. A mature brain takes time to form. Before it's "fully cooked" the brain is vulnerable to any kind of disruption. The frontal lobe where planning, impulse, and memory is processed, isn't finished making new neurons until the age of about 24. Given what we know about brain development, it's a safe bet that eventually we all mature enough to make good decisions, but the teen years, however long they last, are still going to be a little rocky.