Smoking marijuana regularly before the age of 16 causes changes in the brain that can impair a young person's ability to focus, learn from mistakes and think abstractly, according to a Harvard study. On brain scans, the youngest pot smokers showed activation in regions of the brain that was not seen in those who started smoking after age 16, suggesting early exposure to marijuana causes neural changes, researchers at Harvard- affiliated McLean Hospital found. Early and habitual users performed more poorly on tests of cognitive functions, including mental flexibility. Research on how marijuana changes a developing brain is important as it's the most frequently used illegal drug in the United States, said study author Staci Gruber, the director of the neuroimaging center at McLean Hospital in Belmont, Mass. Almost 16 percent of eighth graders have tried marijuana, and that number rises to 42 percent by 12th grade, a 2009 study from the National Institute on Drug Abuse found. Chronic, early users of marijuana 'make repetitive incorrect responses despite the fact I'm telling them they're wrong,' said Gruber, who is also an assistant professor of psychology at Harvard University. 'That's called 'cognitive inflexibility' and you see it in babies.' The research, presented Monday at the Society for Neuroscience's meeting in San Diego, also found that the group that started earlier smoked more pot more often than those who started later. People who began smoking before age 16 had 25.1 smokes a week, compared with 12.1 in those who began later, the study showed. The early-onset group smoked almost three times as many grams a week, Gruber said. The study compared 33 marijuana smokers to 26 healthy controls. Participants were given a battery of tests designed to measure executive function, a term for brain processes responsible for abilities and behaviors involving abstract thinking, decision making, cognitive flexibility and correcting mistakes. The results showed those who started using marijuana before age 16 made twice as many mistakes on tests of executive function than those who began later. The research didn't examine those people who had started smoking early and stopped, although those people begin to look more like non-smokers in other studies, Gruber said. 'The developing brain is vulnerable,' Gruber said. 'If you were to legalize it, you need to keep in mind marijuana is not entirely benign. Regulations and guidelines are needed anyway given the sheer number of Americans using it.' California voters earlier this month rejected a ballot measure that would have legalized marijuana for personal use. 