According to a study in the Proceedings of the National Academy of Sciences, early age marijuana use may have long-term effects on the brain's makeup, including gray matter and IQ. Researchers found that people who smoked marijuana beginning at age 14 showed less gray matter in the decision-making part of the brain than nonusers. These changes were noted to be more pronounced the younger the individual began using marijuana, as adolescents' brains are maturing during this period. The same scientists also found regular users to have lower IQs than the nonusers in the study, though this is more of a correlation than a causation at this point in time.
Click here to read the full article by Saundra Young and Matthew Stucker for CNN.