People who start smoking marijuana at a young age are more likely to perform poorly at cognitive tasks than those who start at a later age, according to a new study by Harvard-affiliated researchers.

The results, released Monday at the annual meeting of the Society for Neuroscience in San Diego, examined the effect of chronic marijuana use on brain function. Those who began using pot before the age of 16 had more trouble focusing, learning from mistakes, and thinking abstractly than those who began afterwards, the study found.

“The more and the earlier you smoke, the worse you perform on these tasks,” said lead author Medical School Assistant Professor Staci A. Gruber, who is director of the Cognitive and Clinical Neuroimaging Core at McLean Hospital.

A total of 33 chronic marijuana smokers and 26 non-smoking control subjects participated in the study. They were asked to complete a series of tests involving “executive function,” a set of abilities that includes planning, abstract thinking, flexibility, and inhibiting inappropriate responses, according to Gruber.

The chronic smokers made repeated errors and showed an inability to maintain focus. The younger group of smokers made twice as many mistakes as the older group and gave fewer correct responses.

The study also revealed that early-onset users smoked three times the amount of marijuana in grams per week and twice as often than late-onset users.

“Marijuana works the same way as alcohol and nicotine,” Gruber said. “Exposure to marijuana prior to 16 will have significant negative results on development, leading to long-term negative ramifications.”

According to a 2009 study from the National Institute on Drug Abuse, nearly 16 percent of eighth graders have tried marijuana. The number rises to 42 percent by 12th grade.

Lester Grinspoon, an associate professor emeritus of psychiatry at the Medical School, expressed doubts about the study: “I’m a little suspicious because so much of it has been wrong in the past,” he said. “After it’s peer reviewed, then I will look at it again so I can be less skeptical.”

Grinspoon said he agrees with the  view that marijuana should be prohibited to young people, but for a different reason.

“We don’t want them smoking marijuana as much as we don’t want them to drink alcohol or use cocaine or ecstasy,” he said, “simply because I don’t think young people have the wisdom yet. I don’t think they are ready for this experience.”