The human brain continues developing until a person is around 25, according to the NPR website. An 18-year-old person is only halfway through the process of brain development and change that begins with puberty.
The prefrontal cortex, which helps with impulse control and organization, is not fully developed until the mid-point of a person's 20s. It is theorized that this incomplete development leads to greater risk-taking among adolescents and young adults. Although brain development ends at around 25 years of age, neural changes can occur for many years beyond that point. PsychCentral states that some areas of the brain have shown the capability to make new connections well into adulthood proper.