(CBS) Call it the pubescent pot problem.
New research shows that adults who smoked marijuana before age 15 have significant problems with attention span, impulse control, and "executive function" - the ability to plan and carry out tasks.
How about people who took up toking later in life? The same research showed they were much less likely to have such difficulties.
For the research, scientists at the Federal University of Sao Paulo in Brazil used a stardard "card sorting" task to evaluate the cognitive ability of 104 long-term cannabis users, including 49 who had started using before age 15. On average, the early tokers had smoked pot for 10.9 years, the Daily Mail reported.
What happened? The early herb enjoyers made more mistakes than those who waited to use weed, as well as the "controls" who didn't smoke marijuana.
"We found that early-onset, but not late-onset, chronic cannabis users had deficits in their cognitive functioning," study author Dr. Maria Alice Fontes said, according to the paper. "Adolescence is a period in which the brain appears to be particularly vulnerable to the neurotoxic effects of cannabis. The brain before the age of 15 is still developing and maturing, so exposure to cannabis during this period may be more harmful."
Marijuana seems to cause both chemical and structural changes in the brain, study co-author Dr. Karen Bolla, associate professor of neurology at Johns Hopkins Bayview Medical Center in Baltimore, told CBS News. She said rapid brain development continues until one's early twenties.
If early pot smoking really is bad for the brain, lots of Americans may be at risk for long-term cognitive difficulties. A 2009 study found that 7 percent of 8th graders and 16 percent of 10th graders had used marijuana in the past month, according to the National Institute on Drug Abuse website.
Marijuana has been linked to other problems with brain function, including difficulties learning new things and recalling recent events, according to the institute.
So what's the new study's take-away message? "Young people need to be educated that their brains are more vulnerable," Dr. Bolla said. "You don't want to be putting in a lot of drugs."
The study was published in the British Journal of Psychiatry.