Correct, your brain is not fully developed until 25-26. Many 18-20 year olds are still visibly children, and they aren't mature enough to make decisions independently. Although 18 year olds have reached the age of legal responsibility for themselves in our country, we still implement restrictive laws on them, such as not being able to smoke, drink, or open a credit card without a cosigner. In the US, university functions like a rite of passage into adulthood. You enter as a child, and with guidance from older adults and development opportunities given to you, you exit as ready to function as a mature, employable adult.