Frequent marijuana use before the age of 18 may cause irreversible harm to a person's intelligence, attention and memory, scientists claim.

Middle-aged people who had started using marijuana in their teens showed an average decline in IQ by about 8 points and quitting pot did not appear to reverse the loss either, according to researchers at Duke University.

However, researchers found that people who waited until after the age of 18, when their brains have fully developed, before trying marijuana did not show similar mental declines.

The latest findings, published in the journal Proceedings of the National Academy of Sciences, are supported by previous studies showing that frequent heavy marijuana use damages the brain.

Researchers assessed more than 1,037 children born in 1972 or 1973 in Dunedin, New Zealand from birth to age 38 in a long-term study. About 5 percent of the participants in the study were considered marijuana-dependent or were using pot more than once a week before the age of 18. 

All participants were given a series of psychological tests at the age of 38 to assess memory, processing speed, reasoning and visual processing. The people who frequently used marijuana as teens scored significantly worse on most of the tests.

Friends and family of participants who frequently used pot as teens were also more likely to report that these persistent cannabis users had attention and memory problems like often losing focus and forgetting to do tasks.

Researchers noted that the decline in IQ among teens that frequently used marijuana cannot be explained by alcohol, other drug use or having less education.

"Marijuana is not harmless, particularly for adolescents," lead researcher Madeline Meier said in a statement.

While eight IQ points may not sound like a lot on a scale where 100 is the average, researchers noted that a loss from an IQ of 100 to 92 represents a drop from being in the 50th percentile to being in the 29th percentile.

Researcher noted that higher IQ is associated with higher education, higher income, better health and a longer life.

"Somebody who loses 8 IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come," Meier said.

The recent study, which is supported by animal studies, is among the first to distinguish between cognitive problems the person might have had before taking up marijuana, and those that were apparently caused by the drug, according to Laurence Steinberg, a Temple University psychologist who was not involved in the research.

Steinberg explained that animal studies involving nicotine, alcohol and cocaine have shown that frequent exposure before the brain is fully developed can lead to more dependence and long-term changes in the brain.

"This study points to adolescence as a time of heightened vulnerability," Steinberg said. "The findings are pretty clear that it is not simply chronic use that causes deficits, but chronic use with adolescent onset," Steinberg said in a statement.

The latest study does not indicate what a safer age for persistent marijuana use might be or what dosage level causes damage.

Researchers noted that after many years of decline among US teens, daily marijuana use is starting to increase in recent years, and for the first time ever U.S. teens are more likely to be smoking pot than tobacco.

"The simple message is that substance use is not healthy for kids," researcher Avshalom Caspi of the Institute of Psychiatry at King's College London said in a statement. "That's true for tobacco, alcohol, and apparently for cannabis."