According to brain experts from Children's Hospital Boston, teen brains are only around 80 percent developed. New research shows that the human brain may not reach its full development until a person's twenties or possibly even the thirties [Source: Graham].
Teen brains are particularly good at learning and remembering new things -- much better than the adult brain. The teen brain's ability to absorb its environment is one reason teens are so influenced by their environments and interested in so many new things. These learning and retaining skills are present for both boys and girls.
The downside to a teen's ability to soak up new stimuli is that it makes him or her more susceptible to the damage drugs and alcohol can have on the brain. Drug and alcohol use really does kill more brain cells in teens that they do in adults. Teens' brains will still have the sluggish effects of a weekend of drinking days later, greatly affecting the performances of teens at school. Worse, teens' brains make them much more vulnerable to becoming addicted than a fully developed adult brain.
The part of the teen brain not yet fully developed is the frontal lobe, which is the part of the brain that manages impulse control, judgment, insight, and emotional control. So when teens engage in risky and/or foolish behavior, it's not just that they don't have life experience to know better, but that their brains have difficulty assessing consequences. Basically, teens have less myelin (the fatty coating that surrounds nerve cells). Nerve cells need myelin to send signals throughout the brain. So in teen brains, the synapses that are supposed to connect the frontal lobe to the rest of the brain are not working as well as they should. As a result, teens can't access those skills based in the frontal lobe easily.