According to a Harvard study, smoking marijuana habitually before age 16 can cause changes in the brain, making it harder for a young person to focus, learn from their mistakes and to think abstractly.
When brain scans were analyzed, activation was seen in regions of the brain of the youngest pot smokers that was not seen in the smokers who started after age 16. Researchers at Harvard-affiliated McLean Hospital found that this indicates early exposure to marijuana can cause neural changes. Early and habitual users of marijuana did not do well on tests of cognitive functions such as mental flexibility.
Study author Staci Gruber, the director of the cognitive and clinical neuroimaging core at McLean Hospital in Belmont, Mass. claims that research on how marijuana changes a developing brain is very important, because it is the most frequently used illegal drug in the United States. A 2009 study from the National Institute on Drug Abuse found that around 16 percent of eighth graders have tried marijuana, and the amount continues to increase to about 42 percent by 12th grade.